The Collaborative International Dictionary of English v.0.48:

Perennial \Per*en"ni*al\ (p[e^]r*[e^]n"n[i^]*al), a. [L.
   perennis that lasts the whole year through; per through +
   annus year. See Per-, and Annual.]
   1. Lasting or continuing through the year; as, perennial
      fountains.
      [1913 Webster]

   2. Continuing without cessation or intermission; perpetual;
      unceasing; never failing.
      [1913 Webster]

            The perennial existence of bodies corporate.
                                                  --Burke.
      [1913 Webster]

   3. (Bot.) Continuing more than two years; as, a perennial
      steam, or root, or plant.
      [1913 Webster]

   Syn: Perpetual; unceasing; never failing; enduring;
        continual; permanent; uninterrupted.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Perennial \Per*en"ni*al\, n. (Bot.)
   A perennial plant; a plant which lives or continues more than
   two years, whether it retains its leaves in winter or not.
   [1913 Webster]

WordNet (r) 3.0 (2006):

perennial
    adj 1: lasting three seasons or more; "the common buttercup is a
           popular perennial plant" [ant: annual, biennial,
           one-year, two-year]
    2: lasting an indefinitely long time; suggesting self-renewal;
       "perennial happiness"
    3: recurring again and again; "perennial efforts to stipulate
       the requirements" [syn: perennial, recurrent, repeated]
    n 1: (botany) a plant lasting for three seasons or more

Moby Thesaurus II by Grady Ward, 1.0:

146 Moby Thesaurus words for "perennial":
   abiding, age-long, aged, amphibian, ancient, angiosperm, annual,
   antique, aquatic plant, articulated, biennial, catenated,
   ceaseless, chattering, chronic, concatenated, connected, constant,
   continual, continued, continuing, continuous, cosmopolite, cutting,
   cyclical, deciduous, deciduous plant, dicot, dicotyledon, direct,
   diuturnal, durable, endless, enduring, ephemeral, eternal,
   ever-blooming, ever-new, evergreen, everlasting, exotic,
   featureless, flowering plant, fungus, gametophyte, gapless,
   gymnosperm, half-hardy, hardy, hydrophyte, immediate, immortal,
   immutable, imperishable, incessant, indeciduous, interminable,
   intransient, inveterate, joined, jointless, lasting, lifelong,
   linked, long-lasting, long-lived, long-standing, long-term,
   longeval, longevous, machine gun, macrobiotic, monocot, monocotyl,
   monotonous, never-ending, never-failing, nonstop, of long duration,
   of long standing, old, oscillating, perdurable, perduring,
   periodic, permanent, perpetual, persistent, persisting, plant,
   polycot, polycotyl, polycotyledon, pulsating, rapid, recurrent,
   regular, remaining, repeated, repetitive, round-the-clock, running,
   seamless, seed plant, seedling, sempervirent, sempiternal, serried,
   smooth, spermatophyte, sporophyte, stable, staccato, staying,
   steadfast, steady, straight, stuttering, sustained, thallophyte,
   timeless, tough, triennial, twenty-four-hour, unbroken, unceasing,
   unchanging, undifferentiated, undying, unending, unfading,
   unfailing, uniform, unintermitted, unintermittent, unintermitting,
   uninterrupted, unrelieved, unremitting, unstopped, unvarying,
   vascular plant, vegetable, vibrating, vital, weed