[1] : The Collaborative International Dictionary of English v.0.48

active \ac"tive\, a. f. actif, l. activus, fr. agere to act.
   1. having the power or quality of acting; causing change;
      communicating action or motion; acting; -- opposed to
      passive, that receives; as, certain active principles;
      the powers of the mind.
      1913 webster

   2. quick in physical movement; of an agile and vigorous body;
      nimble; as, an active child or animal.
      1913 webster

            active and nervous was his gait.      --wordsworth.
      1913 webster

   3. in action; actually proceeding; working; in force; --
      opposed to quiescent, dormant, or extinct; as,
      active laws; active hostilities; an active volcano.
      1913 webster

   4. given to action; constantly engaged in action; energetic;
      diligent; busy; -- opposed to dull, sluggish,
      indolent, or inert; as, an active man of business;
      active mind; active zeal.
      1913 webster

   5. requiring or implying action or exertion; -- opposed to
      sedentary or to tranquil; as, active employment or
      service; active scenes.
      1913 webster

   6. given to action rather than contemplation; practical;
      operative; -- opposed to speculative or theoretical;
      as, an active rather than a speculative statesman.
      1913 webster

   7. brisk; lively; as, an active demand for corn.
      1913 webster

   8. implying or producing rapid action; as, an active disease;
      an active remedy.
      1913 webster

   9. gram.
      a applied to a form of the verb; -- opposed to
          passive. see active voice, under voice.
      b applied to verbs which assert that the subject acts
          upon or affects something else; transitive.
      c applied to all verbs that express action as distinct
          from mere existence or state.
          1913 webster

   active capital, active wealth, money, or property that
      may readily be converted into money.
      1913 webster

   syn: agile; alert; brisk; vigorous; nimble; lively; quick;
        sprightly; prompt; energetic.
        1913 webster
see also:
passive quiescent dormant extinct dull sluggish 
indolent inert sedentary tranquil speculative 
theoretical active voice voice active capital active wealth 

[2] : WordNet (r) 2.0

active
     adj 1: tending to become more severe or wider in scope; "active
            tuberculosis" ant: inactive
     2: engaged in or ready for military or naval operations; "on
        active duty"; "the platoon is combat-ready"; "review the
        fighting forces" syn: combat-ready, fightinga
     3: disposed to take action or effectuate change; "a director
        who takes an active interest in corporate operations"; "an
        active antagonism"; "he was active in drawing attention to
        their grievances" ant: passive
     4: taking part in an activity; "an active member of the club";
        "he was politically active"; "the participating
        organizations" syn: participating
     5: characterized by energetic activity; "an active toddler";
        "active as a gazelle"; "an active man is a man of action"
        ant: inactive
     6: exerting influence or producing an effect; "an active
        ingredient" ant: inactive
     7: full of activity or engaged in continuous activity; "an
        active seaport"; "an active bond market"; "an active
        account" ant: inactive
     8: in operation; "keep hope alive"; "the tradition was still
        alive"; "an active tradition" syn: alivep
     9: of the sun; characterized by a high level activity in
        sunspots and flares and radio emissions ant: quiet
     10: expressing that the subject of the sentence has the semantic
         function of actor: "hemingway favors active
         constructions" ant: passive
     11: expressing action rather than a state of being; used of
         verbs e.g. `to run' and participial adjectives e.g.
         `running' in `running water' syn: dynamic ant: stative
     12: of e.g. volcanos erupting or liable to erupt; "active
         volcanos" ant: dormant, extinct
     13: producing activity or change ant: counteractive
     14: engaged in full-time work; "active duty"; "though past
         retirement age he is still active in his profession"
         ant: inactive
     15: being in physical motion; "active fish in the aquarium"
     16: characterized by energetic bodily activity; "tennis is an
         active sport"; "a very physical dance performance" syn:
         physical
     n 1: chemical agent capable of activity syn: active agent
     2: the voice used to indicate that the grammatical subject of
        the verb is performing the action or causing the happening
        denoted by the verb; "`the boy threw the ball' uses the
        active voice" syn: active voice ant: passive voice
     3: a person devoted to the active life ant: contemplative
see also:
inactive combat-ready fightinga passive participating alivep 
quiet dynamic stative dormant extinct 
counteractive physical active agent active voice passive voice 
contemplative 

[3] : Moby Thesaurus II by Grady Ward, 1.0

157 moby thesaurus words for "active":
   acting, active voice, activist, activistic, acute, aggressive,
   agile, alert, alive, animated, assiduous, at work, bouncing,
   bouncy, breezy, brisk, bubbly, bustling, busy, catty, chipper,
   compliant, conforming, conscientious, constant, devoted, devout,
   diligent, driving, duteous, dutiful, dynamic, ebullient, effective,
   effectual, effervescent, efficacious, efficient, energetic,
   enterprising, enthusiastic, expeditious, faithful, flexible,
   forceful, forcible, frisky, full, full of go, full of life,
   full of pep, functional, functioning, go-go, going, going on,
   graceful, hearty, hyperactive, impelling, impetuous, in exercise,
   in force, in hand, in motion, in operation, in play, in practice,
   in process, in the works, inaction, incisive, industrious,
   influential, intense, keen, kinetic, live, lively, living, loyal,
   lusty, medio-passive, mercurial, meticulous, mettlesome, middle,
   middle voice, militant, mindful, mobile, motile, motivational,
   motive, motor, moving, nimble, observant, occupied, on foot,
   on the fire, on the go, on the move, ongoing, operating,
   operational, operative, passive, passive voice, peppy, perky, pert,
   physical, potent, powerful, practicing, prompt, propellant,
   propelling, punctilious, punctual, quick, quicksilver, ready,
   reflexive, regardful, robust, running, rushing, scrupulous,
   smacking, snappy, spanking, spirited, sprightly, spry, stirring,
   strenuous, strong, supple, take-charge, take-over, transitional,
   traveling, trenchant, true, vibrant, vigorous, vivacious, vivid,
   voice, wide-awake, working, zestful, zesty, zingy, zippy

[4] : Bouvier's Law Dictionary, Revised 6th Ed (1856)

active. the opposite, of passive. we say active debts, or debts due to us;
passive debts are those we owe.

Active >> A Words
Active, definition of term: Active
active_pag1.html

Active Voice >> A Words
Active Voice, definition of term: Active Voice
active+voice_pag1.html

Active Verb >> A Words
Active Verb, definition of term: Active Verb
active+verb_pag1.html