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