Aa is a genus of plants of the family Orchidaceae. Species in this genus can be found growing terrestrially in cold habitats near the snowline in the Andes and also in Costa Rica; they are usually found close to small streams. The elongated inflorescence grows from a basal rosette of leaves, terminating in a small white non-resupinate flower. This lip is fringed and hood-shaped. The flower gives off a pungent smell that attracts flies. This genus has often been included in the orchid genus Altensteinia.