The Collaborative International Dictionary of English v.0.48:

Constant \Con"stant\ (k[o^]n"stant), a. [L. onstans, -antis, p.
   pr. of constare to stand firm, to be consistent; con- + stare
   to stand: cf. F. constant. See Stand and cf. Cost, v. t.]
   1. Firm; solid; fixed; immovable; -- opposed to fluid.
      [Obs.]
      [1913 Webster]

            If . . . you mix them, you may turn these two fluid
            liquors into a constant body.         --Boyle.
      [1913 Webster]

   2. Not liable, or given, to change; permanent; regular;
      continuous; continually recurring; steadfast; faithful;
      not fickle. Opposite of changeable and variable.
      [1913 Webster]

            Both loving one fair maid, they yet remained
            constant friends.                     --Sir P.
                                                  Sidney.
      [1913 Webster]

            I am constant to my purposes.         --Shak.
      [1913 Webster]

            His gifts, his constant courtship, nothing gained.
                                                  --Dryden.
      [1913 Webster]

            Onward the constant current sweeps.   --Longfellow.
      [1913 Webster]

   3. (Math. & Physics) Remaining unchanged or invariable, as a
      quantity, force, law, etc.
      [1913 Webster]

   4. Consistent; logical. [Obs.] --Shak.

   Syn: Fixed; steadfast; unchanging; permanent; unalterable;
        immutable; invariable; perpetual; continual; resolute;
        firm; unshaken; determined.

   Usage: Constant, Continual, Perpetual. These words are
          sometimes used in an absolute and sometimes in a
          qualified sense. Constant denotes, in its absolute
          sense, unchangeably fixed; as, a constant mind or
          purpose. In its qualified sense, it marks something as
          a "standing" fact or occurence; as, liable to constant
          interruptions; constantly called for. Continual, in
          its absolute sense, coincides with continuous. See
          Continuous. In its qualified sense, it describes a
          thing as occuring in steady and rapid succession; as,
          a round of continual calls; continually changing.
          Perpetual denotes, in its absolute sense, what
          literally never ceases or comes to an end; as,
          perpetual motion. In its qualified sense, it is used
          hyperbolically, and denotes that which rarely ceases;
          as, perpetual disturbance; perpetual noise; perpetual
          intermeddling.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Constant \Con"stant\, n.
   1. That which is not subject to change; that which is
      invariable.
      [1913 Webster]

   2. (Math.) A quantity that does not change its value; -- used
      in countradistinction to variable.
      [1913 Webster]

   3. (Astron.) A number whose value, when ascertained (as by
      observation) and substituted in a general mathematical
      formula expressing an astronomical law, completely
      determines that law and enables predictions to be made of
      its effect in particular cases.
      [Webster 1913 Suppl.]

   4. (Physics) A number expressing some property or condition
      of a substance or of an instrument of precision; as, the
      dielectric constant of quartz; the collimation constant of
      a transit instrument.
      [Webster 1913 Suppl.]

   5. (Computers) a data structure that does not change during
      the course of execution of a program. It may be a number,
      a string, or a more complex data structure; -- contrasted
      with variable.
      [PJC]

   Aberration constant, or Constant of aberration (Astron.),
      a number which by substitution in the general formula for
      aberration enables a prediction to be made of the effect
      of aberration on a star anywhere situated. Its value is
      20[sec].47.

   Absolute constant (Math.), one whose value is absolutely
      the same under all circumstances, as the number 10, or any
      numeral.

   Arbitrary constant, an undetermined constant in a
      differential equation having the same value during all
      changes in the values of the variables.

   Gravitation constant (Physics), the acceleration per unit
      of time produced by the attraction of a unit of mass at
      unit distance. When this is known the acceleration
      produced at any distance can be calculated.

   Solar constant (Astron.), the quantity of heat received by
      the earth from the sun in a unit of time. It is, on the C.
      G. S. system, 0.0417 small calories per square centimeter
      per second. --Young. [Webster 1913 Suppl.]

   Constant of integration (Math.), an undetermined constant
      added to every result of integration.
      [1913 Webster + Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

constant
    adj 1: unvarying in nature; "maintained a constant temperature";
           "principles of unvarying validity" [syn: changeless,
           constant, invariant, unvarying]
    2: steadfast in purpose or devotion or affection; "a man
       constant in adherence to his ideals"; "a constant lover";
       "constant as the northern star" [ant: inconstant]
    3: uninterrupted in time and indefinitely long continuing; "the
       ceaseless thunder of surf"; "in constant pain"; "night and
       day we live with the incessant noise of the city"; "the
       never-ending search for happiness"; "the perpetual struggle
       to maintain standards in a democracy"; "man's unceasing
       warfare with drought and isolation"; "unremitting demands of
       hunger" [syn: ceaseless, constant, incessant, never-
       ending, perpetual, unceasing, unremitting]
    n 1: a quantity that does not vary [syn: constant, constant
         quantity, invariable]
    2: a number representing a quantity assumed to have a fixed
       value in a specified mathematical context; "the velocity of
       light is a constant"

Moby Thesaurus II by Grady Ward, 1.0:

336 Moby Thesaurus words for "constant":
   abiding, accordant, active, age-long, aged, ageless, alike,
   ancient, antique, ardent, articulated, assiduous, atom,
   atomic mass, atomic number, atomic weight, automatic, balanced,
   beaten, catenated, ceaseless, changeless, chattering, chronic,
   clinging, close, coeternal, colorfast, committed, compliant,
   concatenated, confirmed, conforming, connected, conscientious,
   consistent, consonant, continual, continued, continuing,
   continuous, correspondent, cyclical, dateless, dedicated,
   deep-dyed, delicate, dependable, determined, devoted, devout,
   diligent, direct, diuturnal, dogged, double-dyed, durable, duteous,
   dutiful, dyed-in-the-wool, endless, enduring, equable, equal,
   eternal, eterne, even, ever-being, ever-durable, ever-during,
   evergreen, everlasting, everliving, exact, express, fadeless,
   faithful, fast, featureless, fine, firm, fixed, flat, flinty,
   frequent, frozen, gapless, habitual, hackneyed, hardy, homogeneous,
   immediate, immemorial, immobile, immovable, immutable, inalterable,
   incessant, incommutable, inconvertible, indefatigable,
   indefeasible, indelible, indestructible, indomitable, industrious,
   inerrable, inerrant, inert, infallible, infinite, inflexible,
   ingrain, ingrained, insistent, insusceptible of change, intact,
   interminable, intransient, intransmutable, invariable, inveterate,
   invincible, inviolate, irretrievable, irreversible, irrevocable,
   joined, jointless, lasting, level, liege, linked, long-lasting,
   long-lived, long-standing, long-term, longeval, longevous, loyal,
   machine gun, macrobiotic, marble-constant, mathematical, measured,
   mechanical, methodic, methodical, meticulous,
   micrometrically precise, microscopic, mindful, monolithic,
   monotonous, never-ceasing, never-ending, never-tiring, nice, noble,
   nonreturnable, nonreversible, nonstop, nonterminating,
   nonterminous, observant, obstinate, of a piece, of long duration,
   of long standing, olamic, ordered, orderly, oscillating, patient,
   patient as Job, perdurable, perduring, perennial, periodic,
   permanent, perpetual, perseverant, persevering, persistent,
   persisting, pertinacious, pinpoint, plodding, plugging, practicing,
   precise, preoccupied, pulsating, punctilious, punctual, quantum,
   quiescent, rapid, rapt, recurrent, recurring, refined, regardful,
   regular, regular as clockwork, relentless, religious,
   religiously exact, remaining, repeated, repetitive, resolute,
   reverseless, rigid, rigorous, robotlike, round-the-clock, routine,
   running, scientific, scientifically exact, scrupulous, seamless,
   sedulous, sempervirent, sempiternal, serried, set, settled, severe,
   single-minded, sleepless, slogging, smooth, solid, sot, square,
   stabile, stable, staccato, static, stationary, staunch, staying,
   steadfast, steady, steely, stereotyped, straight, strict, stubborn,
   stuttering, subtle, sustained, systematic, tenacious, tested,
   timeless, tireless, torpid, tough, tried, tried and true, trite,
   true, true-blue, trusty, twenty-four-hour, unabating, unalterable,
   unalterative, unaltered, unbending, unbroken, unceasing,
   unchangeable, unchanged, unchanging, unchecked, unconquerable,
   undaunted, undeflectable, undestroyed, undeviating,
   undifferentiated, undiscouraged, undiversified, undrooping,
   unending, unerring, unfading, unfailing, unfaltering, unflagging,
   unflappable, unflinching, uniform, unintermitted, unintermittent,
   unintermitting, uninterrupted, unmodifiable, unmovable, unnodding,
   unrelaxing, unrelenting, unrelieved, unremitting, unrestorable,
   unreturnable, unruffled, unshakable, unshakeable, unshaken,
   unshifting, unsleeping, unstopped, unsusceptible, unswerving,
   untiring, unvariable, unvaried, unvarying, unwavering, unwearied,
   unwearying, unwinking, unyielding, utterly attentive, valence,
   vibrating, vital, weariless, well-trodden, well-worn,
   without end