A catamaran (often referred to as a "kat") is a multihull boat with two parallel hulls of equal size.

Catamarans have two distinct primary performance characteristics that distinguish them from displacement monohulls: less resistance to passage through the water and greater stability (initial resistance to capsizing). The choice between a monohull and a catamaran configuration takes into account carrying capacity, speed and efficiency.

It is a geometrically stabilised boat, which derives its stability from its wide beam and not from a ballast keel as on a monohull. Catamarans generally have a smaller hull volume, displacement and draught than monohulls of comparable length. The two hulls together also often have lower hydrodynamic resistance than comparable monohulls, requiring less propulsion force from sails or engines. The wider position of the catamaran on the water can reduce both heel and wave-induced motion compared to a monohull and result in less swell.