Mardi Gras (UK: /ˌmɑːrdi ˈɡrɑː/, US: /ˈmɑːrdi ɡrɑː/; also known as Shrove Tuesday) is the final day of Carnival (also known as Shrovetide or Fastelavn); it thus falls on the day before the beginning of Lent on Ash Wednesday. Mardi Gras is French for "Fat Tuesday", referring to it being the last day of consuming rich, fatty foods, most notably red meat, in preparation for the Christian fasting season of Lent, during which such foods are avoided.

Related popular practices are associated with Carnival celebrations before the fasting and religious obligations associated with the penitential season of Lent. In countries such as the United Kingdom, Mardi Gras is more usually known as Pancake Day or (traditionally) Shrove Tuesday, derived from the word shrive, meaning "to administer the sacrament of confession to; to absolve".

During the liturgical season of Lent, some Christians abstain from the consumption of certain foods such as meat, eggs, dairy products, and alcoholic beverages. Most Christian denominations observe the tradition of Lent; exceptions include many churches within the Anabaptist, Baptist, Methodist, and Reformed traditions. Shrovetide provided Christians with the opportunity to use up these foods prior to the start of the 40-day fasting season of Lent.

Prior to the 6th century, Lent was normatively observed through the practice of the Black Fast, which enjoins fasting from food and liquids, with the allowance of one vegetarian meal and water after sunset. The tradition of pancake breakfasts during Shrovetide, as well as that of pancake races, owes itself to this practice of "using up the surplus eggs, milk and butter" prior to Lent.

The specific tradition of eating pancakes is said to have roots in Slavic paganism, with this practice being co-opted into Christian ritual. Specifically the pancake was said to symbolise the returning of the sun as spring approached.

In many Christian parish churches, both Protestant and Roman Catholic, a popular Shrove Tuesday tradition is the ringing of the church bells (on this day, the toll is known as the Shriving Bell) "to call the faithful to confession before the solemn season of Lent" and for people to "begin frying their pancakes". As such, a hallmark of Shrovetide is the opportunity for a last round of merrymaking associated with Mardis Gras before the start of the somber Lenten season. The last day of Shrovetide, Mardi Gras (Fat Tuesday), is named as such "because people felt bloated having eaten up all the rich foods before Lent" in order to prepare for the coming season of repentance.

The festival season varies from city to city; Mardis Gras often refers to the last day of Shrovetide (or Fastelavn or Carnival), thus being synonymous with Shrove Tuesday. Some traditions, such as the one in New Orleans, Louisiana, consider Mardi Gras to stretch the entire period from Twelfth Night (the last night of Christmas which begins Epiphany) to Ash Wednesday. Others treat the final three-day period before Ash Wednesday as the Mardi Gras.

In Mobile, Alabama, Mardi Gras–associated social events begin in November, followed by mystic society balls New Year's Eve, followed by parades and balls in January and February, celebrating up to midnight before Ash Wednesday. In earlier times, parades were held on New Year's Day. Carnival (or Fastelavn or Shrovetide) is an important celebration in Lutheran, Anglican and Catholic European nations.