The mess (also called a mess deck aboard ships) is a designated area where military personnel socialize, eat and (in some cases) live. The term is also used to indicate the groups of military personnel who belong to separate messes, such as the officers' mess, the chief petty officer mess, and the enlisted mess. In some civilian societies this military usage has been extended to the eating arrangements of other disciplined services such as fire fighting and police forces.

The root of mess is the Old French mes, "portion of food" (cf. modern French mets), drawn from the Latin verb mittere, meaning "to send" and "to put" (cf. modern French mettre), the original sense being "a course of a meal put on the table"; cfr. also the modern Italian portata with the same meaning, past participle of portare, to bring. This sense of mess, which appeared in English in the 13th century, was often used for cooked or liquid dishes in particular, as in the "mess of pottage" (porridge or soup) or Eton mess.