The Collaborative International Dictionary of English v.0.48:

Limerick \Lim"er*ick\ (l[i^]m"[~e]r*[i^]k), n. [Said to be from
   a song with the same verse construction, current in Ireland,
   the refrain of which contains the place name Limerick.]
   A humorous, often nonsensical, and sometimes risq['e] poem of
   five anapestic lines, of which lines 1, 2, and 5 are of three
   feet, and rhyme, and lines 3 and 4 are of two feet, and
   rhyme.

   Note: It often begins with "There once was a . . ." or "There
         was a . . ."; as

               There was a young lady, Amanda,
               Whose Ballades Lyriques were quite fin de
               Si[`e]cle, I deem
               But her Journal Intime
               Was what sent her papa to Uganda.
         [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

Limerick
    n 1: port city in southwestern Ireland
    2: a humorous verse form of 5 anapestic lines with a rhyme
       scheme aabba