MTT is an abbreviation for Multi-Table Tournament, and indicates a tournament where play is occurring across multiple tables at once. MTTs are named as such to distinguish them from Single-Table Tournaments (STTs).
In Multi-Table Tournaments, tables are slowly combined during the course of play as players are eliminated. Eventually, the tournament is reduced to a single table.
Most live tournaments are MTTs, with the exception of satellites, which tend to be STTs. Online poker rooms offer a staggering array of MTTs on any given day. Most are scheduled and begin at a set time, but some are configured to begin when a set number of entrants register. The latter are referred to as MTT Sit and Go’s, named for the fact that the tournament ‘goes’ when enough players are seated.