Mahayana (Sanskrit: महायान, mahāyāna literally 'Great Vehicle') is one of two major divisions of Buddhism, along with Theravada. In this sense, Mahayana Buddhism is distinguished primarily by its recognition of the Mahayana sutras, which Theravadins reject as spurious.
Among its adherents, "Mahayana" also refers to a level of spiritual motivation or realization based on bodhicitta--the altruistic aspiration to achieve enlightenment not only for one's own sake, but for the sake of all sentient beings. The term contrasts with "Hinayana" (the "Small Vehicle"); and in some forms of Tibetan Buddhism, with Vajrayana (the "Diamond Vehicle," i.e., tantric Buddhism), though this is Mahayanist in terms of motivation.
Note on usage: The term "Hinayana" tends to be received as a pejorative among adherents of the Theravada tradition, and is therefore often avoided. A useful alternative, for historical contexts when "Theravada" is too narrow, is Nikaya Buddhism. In theological discussions, one may refer to followers of the Shravaka ("Hearer") or Pratyekabuddha ("Solitary Realizer") path.