The Collaborative International Dictionary of English v.0.48:

Line \Line\, n. [OE. line, AS. l[imac]ne cable, hawser, prob.
   from L. linea a linen thread, string, line, fr. linum flax,
   thread, linen, cable; but the English word was influenced by
   F. ligne line, from the same L. word linea. See Linen.]
   1. A linen thread or string; a slender, strong cord; also, a
      cord of any thickness; a rope; a hawser; as, a fishing
      line; a line for snaring birds; a clothesline; a towline.
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            Who so layeth lines for to latch fowls. --Piers
                                                  Plowman.
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   2. A more or less threadlike mark of pen, pencil, or graver;
      any long mark; as, a chalk line.
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   3. The course followed by anything in motion; hence, a road
      or route; as, the arrow descended in a curved line; the
      place is remote from lines of travel.
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   4. Direction; as, the line of sight or vision.
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   5. A row of letters, words, etc., written or printed; esp., a
      row of words extending across a page or column.
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   6. A short letter; a note; as, a line from a friend.
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   7. (Poet.) A verse, or the words which form a certain number
      of feet, according to the measure.
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            In the preceding line Ulysses speaks of Nausicaa.
                                                  --Broome.
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   8. Course of conduct, thought, occupation, or policy; method
      of argument; department of industry, trade, or
      intellectual activity.
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            He is uncommonly powerful in his own line, but it is
            not the line of a first-rate man.     --Coleridge.
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   9. (Math.) That which has length, but not breadth or
      thickness.
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   10. The exterior limit of a figure, plat, or territory;
       boundary; contour; outline.
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             Eden stretched her line
             From Auran eastward to the royal towers
             Of great Seleucia.                   --Milton.
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   11. A threadlike crease marking the face or the hand; hence,
       characteristic mark.
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             Though on his brow were graven lines austere.
                                                  --Byron.
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             He tipples palmistry, and dines
             On all her fortune-telling lines.    --Cleveland.
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   12. Lineament; feature; figure. "The lines of my boy's face."
       --Shak.
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   13. A straight row; a continued series or rank; as, a line of
       houses, or of soldiers; a line of barriers.
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             Unite thy forces and attack their lines. --Dryden.
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   14. A series or succession of ancestors or descendants of a
       given person; a family or race; as, the ascending or
       descending line; the line of descent; the male line; a
       line of kings.
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             Of his lineage am I, and his offspring
             By very line, as of the stock real.  --Chaucer.
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   15. A connected series of public conveyances, and hence, an
       established arrangement for forwarding merchandise, etc.;
       as, a line of stages; an express line.
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   16. (Geog.)
       (a) A circle of latitude or of longitude, as represented
           on a map.
       (b) The equator; -- usually called the line, or
           equinoctial line; as, to cross the line.
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   17. A long tape, or a narrow ribbon of steel, etc., marked
       with subdivisions, as feet and inches, for measuring; a
       tapeline.
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   18. (Script.)
       (a) A measuring line or cord.
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                 He marketh it out with a line.   --Is. xliv.
                                                  13.
       (b) That which was measured by a line, as a field or any
           piece of land set apart; hence, allotted place of
           abode.
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                 The lines are fallen unto me in pleasant
                 places; yea, I have a goodly heritage. --Ps.
                                                  xvi. 6.
       (c) Instruction; doctrine.
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                 Their line is gone out through all the earth.
                                                  --Ps. xix. 4.
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   19. (Mach.) The proper relative position or adjustment of
       parts, not as to design or proportion, but with reference
       to smooth working; as, the engine is in line or out of
       line.
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   20. The track and roadbed of a railway; railroad.
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   21. (Mil.)
       (a) A row of men who are abreast of one another, whether
           side by side or some distance apart; -- opposed to
           column.
       (b) The regular infantry of an army, as distinguished
           from militia, guards, volunteer corps, cavalry,
           artillery, etc.
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   22. (Fort.)
       (a) A trench or rampart.
       (b) pl. Dispositions made to cover extended positions,
           and presenting a front in but one direction to an
           enemy.
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   23. pl. (Shipbuilding) Form of a vessel as shown by the
       outlines of vertical, horizontal, and oblique sections.
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   24. (Mus.) One of the straight horizontal and parallel
       prolonged strokes on and between which the notes are
       placed.
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   25. (Stock Exchange) A number of shares taken by a jobber.
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   26. (Trade) A series of various qualities and values of the
       same general class of articles; as, a full line of
       hosiery; a line of merinos, etc. --McElrath.
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   27. The wire connecting one telegraphic station with another,
       or the whole of a system of telegraph wires under one
       management and name.
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   28. pl. The reins with which a horse is guided by his driver.
       [U. S.]
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   29. A measure of length; one twelfth of an inch.
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   Hard lines, hard lot. --C. Kingsley. [See Def. 18.]

   Line breeding (Stockbreeding), breeding by a certain family
      line of descent, especially in the selection of the dam or
      mother.

   Line conch (Zool.), a spiral marine shell (Fasciolaria
      distans), of Florida and the West Indies. It is marked by
      narrow, dark, revolving lines.

   Line engraving.
       (a) Engraving in which the effects are produced by lines
           of different width and closeness, cut with the burin
           upon copper or similar material; also, a plate so
           engraved.
       (b) A picture produced by printing from such an
           engraving.

   Line of battle.
       (a) (Mil. Tactics) The position of troops drawn up in
           their usual order without any determined maneuver.
       (b) (Naval) The line or arrangement formed by vessels of
           war in an engagement.

   Line of battle ship. See Ship of the line, below.

   Line of beauty (Fine Arts),an abstract line supposed to be
      beautiful in itself and absolutely; -- differently
      represented by different authors, often as a kind of
      elongated S (like the one drawn by Hogarth).

   Line of centers. (Mach.)
       (a) A line joining two centers, or fulcra, as of wheels
           or levers.
       (b) A line which determines a dead center. See Dead
           center, under Dead.

   Line of dip (Geol.), a line in the plane of a stratum, or
      part of a stratum, perpendicular to its intersection with
      a horizontal plane; the line of greatest inclination of a
      stratum to the horizon.

   Line of fire (Mil.), the direction of fire.

   Line of force (Physics), any line in a space in which
      forces are acting, so drawn that at every point of the
      line its tangent is the direction of the resultant of all
      the forces. It cuts at right angles every equipotential
      surface which it meets. Specifically (Magnetism), a line
      in proximity to a magnet so drawn that any point in it is
      tangential with the direction of a short compass needle
      held at that point. --Faraday.

   Line of life (Palmistry), a line on the inside of the hand,
      curving about the base of the thumb, supposed to indicate,
      by its form or position, the length of a person's life.

   Line of lines. See Gunter's line.

   Line of march. (Mil.)
       (a) Arrangement of troops for marching.
       (b) Course or direction taken by an army or body of
           troops in marching.

   Line of operations, that portion of a theater of war which
      an army passes over in attaining its object. --H. W.
      Halleck.

   Line of sight (Firearms), the line which passes through the
      front and rear sight, at any elevation, when they are
      sighted at an object.

   Line tub (Naut.), a tub in which the line carried by a
      whaleboat is coiled.

   Mason and Dixon's line, Mason-Dixon line, the boundary
      line between Pennsylvania and Maryland, as run before the
      Revolution (1764-1767) by two English astronomers named
      Charles Mason and Jeremiah Dixon. In an extended sense,
      the line between the free and the slave States; as, below
      the Mason-Dixon line, i.e. in the South.

   On the line,
       (a) on a level with the eye of the spectator; -- said of
           a picture, as hung in an exhibition of pictures.
       (b) at risk (dependent upon success) in a contest or
           enterprise; as, the survival of the company is on the
           line in this project.

   Right line, a straight line; the shortest line that can be
      drawn between two points.

   Ship of the line, formerly, a ship of war large enough to
      have a place in the line of battle; a vessel superior to a
      frigate; usually, a seventy-four, or three-decker; --
      called also line of battle ship or battleship.
      --Totten.

   To cross the line, to cross the equator, as a vessel at
      sea.

   To give a person line, to allow him more or less liberty
      until it is convenient to stop or check him, like a hooked
      fish that swims away with the line.

   Water line (Shipbuilding), the outline of a horizontal
      section of a vessel, as when floating in the water.
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WordNet (r) 3.0 (2006):

line of sight
    n 1: an imaginary straight line along which an observer looks
         [syn: line of sight, line of vision]