The Collaborative International Dictionary of English v.0.48:

Perpetual \Per*pet"u*al\, a. [OE. perpetuel, F. perp['e]tuel,
   fr. L. perpetualis, fr. perpetuus continuing throughout,
   continuous, fr. perpes, -etis, lasting throughout.]
   Neverceasing; continuing forever or for an unlimited time;
   unfailing; everlasting; continuous.
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         Unto the kingdom of perpetual night.     --Shak.
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         Perpetual feast of nectared sweets.      --Milton.
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   Circle of perpetual apparition, or Circle of perpetual
   occultation. See under Circle.

   Perpetual calendar, a calendar so devised that it may be
      adjusted for any month or year.

   Perpetual curacy (Ch. of Eng.), a curacy in which all the
      tithes are appropriated, and no vicarage is endowed.
      --Blackstone.

   Perpetual motion. See under Motion.

   Perpetual screw. See Endless screw, under Screw.
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   Syn: Continual; unceasing; endless; everlasting; incessant;
        constant; eternal. See Constant.
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The Collaborative International Dictionary of English v.0.48:

Tax \Tax\, n. [F. taxe, fr. taxer to tax, L. taxare to touch,
   sharply, to feel, handle, to censure, value, estimate, fr.
   tangere, tactum, to touch. See Tangent, and cf. Task,
   Taste.]
   1. A charge, especially a pecuniary burden which is imposed
      by authority. Specifically:
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      (a) A charge or burden laid upon persons or property for
          the support of a government.
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                A farmer of taxes is, of all creditors,
                proverbially the most rapacious.  --Macaulay.
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      (b) Especially, the sum laid upon specific things, as upon
          polls, lands, houses, income, etc.; as, a land tax; a
          window tax; a tax on carriages, and the like.

   Note: Taxes are annual or perpetual, direct or
         indirect, etc.
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      (c) A sum imposed or levied upon the members of a society
          to defray its expenses.
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   2. A task exacted from one who is under control; a
      contribution or service, the rendering of which is imposed
      upon a subject.
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   3. A disagreeable or burdensome duty or charge; as, a heavy
      tax on time or health.
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   4. Charge; censure. [Obs.] --Clarendon.
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   5. A lesson to be learned; a task. [Obs.] --Johnson.
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   Tax cart, a spring cart subject to a low tax. [Eng.]
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   Syn: Impost; tribute; contribution; duty; toll; rate;
        assessment; exaction; custom; demand.
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WordNet (r) 3.0 (2006):

perpetual
    adj 1: continuing forever or indefinitely; "the ageless themes
           of love and revenge"; "eternal truths"; "life
           everlasting"; "hell's perpetual fires"; "the unending
           bliss of heaven" [syn: ageless, aeonian, eonian,
           eternal, everlasting, perpetual, unending,
           unceasing]
    2: 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]

Moby Thesaurus II by Grady Ward, 1.0:

184 Moby Thesaurus words for "perpetual":
   abiding, age-long, aged, ageless, all-comprehensive, all-inclusive,
   all-knowing, all-powerful, all-seeing, all-wise, almighty, ancient,
   antique, boundless, ceaseless, changeless, chattering, chronic,
   coeternal, constant, continual, continuing, continuous, countless,
   creating, creative, dateless, diuturnal, durable, endless,
   enduring, eternal, eternally the same, eterne, ever-being,
   ever-durable, ever-during, evergreen, everlasting, everliving,
   exhaustless, extending everywhere, firm, fixed, frozen, glorious,
   good, hallowed, hardy, highest, holy, illimitable, illimited,
   immeasurable, immemorial, immense, immobile, immortal, immutable,
   incalculable, incessant, incomprehensible, indestructible,
   inexhaustible, infinite, infinitely continuous, innumerable,
   intact, interminable, interminate, intransient, invariable,
   inveterate, inviolate, just, lasting, limitless, long-lasting,
   long-lived, long-standing, long-term, longeval, longevous, loving,
   luminous, machine gun, macrobiotic, majestic, making, measureless,
   merciful, never-ceasing, never-ending, no end of, nonstop,
   nonterminating, nonterminous, numinous, of long duration,
   of long standing, olamic, omnipotent, omnipresent, omniscient, one,
   oscillating, perdurable, perduring, perennial, permanent,
   persistent, persisting, pulsating, quiescent, radiant, rapid,
   recurrent, regular, remaining, repeated, repetitive, rigid, sacred,
   sempervirent, sempiternal, shaping, shoreless, solid, sovereign,
   stable, staccato, static, stationary, staying, steadfast, steady,
   stuttering, sumless, supreme, sustained, termless, timeless,
   torpid, tough, ubiquitous, unaltered, unbounded, unbroken,
   unceasing, unchangeable, unchanged, unchanging, unchecked,
   uncircumscribed, undefined, undestroyed, undeviating, unending,
   unfading, unfailing, unfathomable, unintermitted, unintermittent,
   unintermitting, uninterrupted, universal, unlimited, unmeasurable,
   unmeasured, unnumbered, unplumbed, unremitting, unshifting,
   unstopped, untold, unvaried, unvarying, vibrating, vital,
   without bound, without end, without limit, without measure,
   without number



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

PERPETUAL. That which is to last without limitation as to time; as, a
perpetual statute, which is one without limit as to time, although not
expressed to be so.