The Collaborative International Dictionary of English v.0.48:

Solid \Sol"id\ (s[o^]l"[i^]d), a. [L. solidus, probably akin to
   sollus whole, entire, Gr. ???: cf. F. solide. Cf.
   Consolidate,Soda, Solder, Soldier, Solemn.]
   1. Having the constituent parts so compact, or so firmly
      adhering, as to resist the impression or penetration of
      other bodies; having a fixed form; hard; firm; compact; --
      opposed to fluid and liquid or to plastic, like
      clay, or to incompact, like sand.
      [1913 Webster]

   2. Not hollow; full of matter; as, a solid globe or cone, as
      distinguished from a hollow one; not spongy; dense;
      hence, sometimes, heavy.
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   3. (Arith.) Having all the geometrical dimensions; cubic; as,
      a solid foot contains 1,728 solid inches.
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   Note: In this sense, cubics now generally used.
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   4. Firm; compact; strong; stable; unyielding; as, a solid
      pier; a solid pile; a solid wall.
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   5. Applied to a compound word whose parts are closely united
      and form an unbroken word; -- opposed to hyphened.
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   6. Fig.: Worthy of credit, trust, or esteem; substantial, as
      opposed to frivolous or fallacious; weighty; firm;
      strong; valid; just; genuine.
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            The solid purpose of a sincere and virtuous answer.
                                                  --Milton.
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            These, wanting wit, affect gravity, and go by the
            name of solid men.                    --Dryden.
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            The genius of the Italians wrought by solid toil
            what the myth-making imagination of the Germans had
            projected in a poem.                  --J. A.
                                                  Symonds.
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   7. Sound; not weakly; as, a solid constitution of body. --I.
      Watts.
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   8. (Bot.) Of a fleshy, uniform, undivided substance, as a
      bulb or root; not spongy or hollow within, as a stem.
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   9. (Metaph.) Impenetrable; resisting or excluding any other
      material particle or atom from any given portion of space;
      -- applied to the supposed ultimate particles of matter.
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   10. (Print.) Not having the lines separated by leads; not
       open.
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   11. United; without division; unanimous; as, the delegation
       is solid for a candidate. [Polit. Cant. U.S.]
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   Solid angle. (Geom.) See under Angle.

   Solid color, an even color; one not shaded or variegated.


   Solid green. See Emerald green
       (a), under Green.

   Solid measure (Arith.), a measure for volumes, in which the
      units are each a cube of fixed linear magnitude, as a
      cubic foot, yard, or the like; thus, a foot, in solid
      measure, or a solid foot, contains 1,728 solid inches.

   Solid newel (Arch.), a newel into which the ends of winding
      stairs are built, in distinction from a hollow newel. See
      under Hollow, a.

   Solid problem (Geom.), a problem which can be construed
      geometrically, only by the intersection of a circle and a
      conic section or of two conic sections. --Hutton.

   Solid square (Mil.), a square body or troops in which the
      ranks and files are equal.
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   Syn: Hard; firm; compact; strong; substantial; stable; sound;
        real; valid; true; just; weighty; profound; grave;
        important.

   Usage: Solid, Hard. These words both relate to the
          internal constitution of bodies; but hardnotes a more
          impenetrable nature or a firmer adherence of the
          component parts than solid. Hard is opposed to soft,
          and solid to fluid, liquid, open, or hollow. Wood is
          usually solid; but some kinds of wood are hard, and
          others are soft.
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                Repose you there; while I [return] to this hard
                house,
                More harder than the stones whereof 't is
                raised.                           --Shak.
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                I hear his thundering voice resound,
                And trampling feet than shake the solid ground.
                                                  --Dryden.
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The Collaborative International Dictionary of English v.0.48:

Solid \Sol"id\, n.
   1. A substance that is held in a fixed form by cohesion among
      its particles; a substance not fluid.
      [1913 Webster]

   2. (Geom.) A magnitude which has length, breadth, and
      thickness; a part of space bounded on all sides.
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   Solid of revolution. (Geom.) See Revolution, n., 5.
      [1913 Webster]

WordNet (r) 3.0 (2006):

solid
    adj 1: characterized by good substantial quality; "solid
           comfort"; "a solid base hit"
    2: of definite shape and volume; firm; neither liquid nor
       gaseous; "ice is water in the solid state" [ant: gaseous,
       liquid]
    3: entirely of one substance with no holes inside; "a solid
       block of wood" [ant: hollow]
    4: of one substance or character throughout; "solid gold";
       "carved out of solid rock"
    5: uninterrupted in space; having no gaps or breaks; "a solid
       line across the page"; "solid sheets of water"
    6: providing abundant nourishment; "a hearty meal"; "good solid
       food"; "ate a substantial breakfast"; "four square meals a
       day" [syn: hearty, satisfying, solid, square,
       substantial]
    7: of good quality and condition; solidly built; "a solid
       foundation"; "several substantial timber buildings" [syn:
       solid, strong, substantial]
    8: not soft or yielding to pressure; "a firm mattress"; "the
       snow was firm underfoot"; "solid ground" [syn: firm,
       solid]
    9: having three dimensions; "a solid object"
    10: impenetrable for the eye; "solid blackness"
    11: financially sound; "the bank is solid and will survive this
        attack"
    12: of a substantial character and not frivolous or superficial;
        "work of solid scholarship"; "based on solid facts"
    13: meriting respect or esteem; "an upstanding member of the
        community" [syn: upstanding, solid]
    14: of the same color throughout; "solid color" [syn: solid,
        self-colored, self-coloured]
    15: acting together as a single undiversified whole; "a solid
        voting bloc" [syn: solid, unanimous, whole]
    n 1: matter that is solid at room temperature and pressure
    2: the state in which a substance has no tendency to flow under
       moderate stress; resists forces (such as compression) that
       tend to deform it; and retains a definite size and shape
       [syn: solid, solidness, solid state]
    3: a three-dimensional shape

The Free On-line Dictionary of Computing (19 January 2023):

SOLID

    An acronym for some principles of good software
   architecture, originally compiled by Robert C. Martin in
   the 1990s.  The letters stand for:

   - SRP Single Responsibility Principle

   - OCP Open Closed Principle

   - LSP Liskov Substitution Principle

   - ISP Interface Segregation Principle

   - DIP Dependency Inversion Principle

   (2017-11-15)