The Collaborative International Dictionary of English v.0.48:

Mardi gras \Mar"di` gras"\, n. [F., literally, fat Tuesday.]
   1. The last day of Carnival; the Tuesday before Ash Wednesday
      (the beginning of Lent and fasting); Shrove Tuesday; -- in
      some cities a great day of carnival and merrymaking; in
      the United States it is especially associated with New
      Orleans.

   Syn: pancake day.
        [1913 Webster]

   2. The series of festival events celebrated on Mardi
      Gras[1].
      [PJC]

WordNet (r) 3.0 (2006):

Mardi Gras
    n 1: the last day before Lent [syn: Mardi Gras, Shrove
         Tuesday, pancake day]
    2: a carnival held in some countries on Shrove Tuesday (the last
       day before Lent) but especially in New Orleans [syn: Mardi
       Gras, Fat Tuesday]