A collective name for the 18 schools of Indian Buddhism that emphasize methods for attaining liberation from suffering. Theravada, practiced throughout Southeast Asia and Sri Lanka, is the only extant school of these 18.
Literally, a "Modest Vehicle of Mind" -- levels and states of mind with a modest motivation - renunciation - and, with modest methods, lead to the modest goal of liberation. Some translators render the term as "Lesser Vehicle."
J Hopkins: Lesser Vehicle/Hīnayāna.