Cremini, also called crimini or brown mushrooms, are brown-capped mushrooms of the species Agaricus bisporus, with a firmer texture and a more pronounced, earthy flavor than white button mushrooms.
Brown Agaricus bisporus was the original strain cultivated in France beginning in the 17th century. The now-common white button mushroom arose later, from a natural white mutation discovered on a Pennsylvania mushroom farm in 1925 that was propagated commercially. White button, cremini, and portobello are all the same species at different strains or maturity: a cremini left to mature further becomes a portobello.
Cremini hold up well to high heat and release less liquid than button mushrooms, making them well suited to sautéing, roasting, and stuffing. Avoid crowding the pan, or they'll steam rather than brown.