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]

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "Mardi Gras":
   Advent, Allhallowmas, Allhallows, Allhallowtide, Annunciation,
   Annunciation Day, Ascension Day, Ash Wednesday, Candlemas,
   Candlemas Day, Carnival, Christmas, Corpus Christi, Easter,
   Easter Monday, Easter Saturday, Easter Sunday, Eastertide,
   Ember days, Epiphany, Good Friday, Halloween, Hallowmas,
   Holy Thursday, Holy Week, Lady Day, Lammas, Lammas Day, Lammastide,
   Lent, Lententide, Martinmas, Maundy Thursday, Michaelmas,
   Michaelmas Day, Michaelmastide, Palm Sunday, Pancake Day,
   Passion Week, Pentecost, Quadragesima, Quadragesima Sunday,
   Saturnalia, Septuagesima, Shrove Tuesday, Trinity Sunday,
   Twelfth-day, Twelfth-tide, Whit-Tuesday, White Sunday, Whitmonday,
   Whitsun, Whitsunday, Whitsuntide, Whitweek, banquet, blowout,
   carnival, do, fair, feast, festival, festive occasion, festivity,
   fete, field day, fiesta, gala, gala affair, gala day, great doings,
   high jinks, jamboree, kermis, party, picnic, waygoose, wayzgoose