Source : Webster's Revised Unabridged Dictionary (1913)

Mardi gras \Mar"di` gras"\, n. [F., literally, fat Tuesday.]
   The last day of Carnival; Shrove Tuesday; -- in some cities a
   great day of carnival and merrymaking.

Source : WordNet®

Mardi Gras
     n 1: the last day before Lent [syn: {Shrove Tuesday}, {pancake
          day}]
     2: the pre-Lenten festival of Shrove Tuesday [syn: {Fat Tuesday}]