The Collaborative International Dictionary of English v.0.48:

Segment \Seg"ment\, n. [L. segmentum, fr. secare to cut, cut
   off: cf. F. segment. See Saw a cutting instrument.]
   1. One of the parts into which any body naturally separates
      or is divided; a part divided or cut off; a section; a
      portion; as, a segment of an orange; a segment of a
      compound or divided leaf.
      [1913 Webster]

   2. (Geom.) A part cut off from a figure by a line or plane;
      especially, that part of a circle contained between a
      chord and an arc of that circle, or so much of the circle
      as is cut off by the chord; as, the segment acb in the
      Illustration.
      [1913 Webster]

   3. (Mach.)
      (a) A piece in the form of the sector of a circle, or part
          of a ring; as, the segment of a sectional fly wheel or
          flywheel rim.
      (b) A segment gear.
          [1913 Webster]

   4. (Biol.)
      (a) One of the cells or division formed by segmentation,
          as in egg cleavage or in fissiparous cell formation.
      (b) One of the divisions, rings, or joints into which many
          animal bodies are divided; a somite; a metamere; a
          somatome.
          [1913 Webster]

   Segment gear, a piece for receiving or communicating
      reciprocating motion from or to a cogwheel, consisting of
      a sector of a circular gear, or ring, having cogs on the
      periphery, or face.

   Segment of a line, the part of a line contained between two
      points on it.

   Segment of a sphere, the part of a sphere cut off by a
      plane, or included between two parallel planes.

   Ventral segment. (Acoustics) See Loor, n., 5.
      [1913 Webster]