A Bodhisattva is a human being who has awakened but put off not being reborn in order to return and help all beings awaken and they make a vow to keep returning for life after life until that happens. The term comes from two Sanskrit words - Bodhi or awakening and Sattva, which means being. It means the same thing in all branches of Buddhism but is emphasised less in Theravada Buddhism than in the Mahayana tradition, where it is a central facet of Buddhist practice.

Bodhisattva is a term *very* loosely used in Buddhism to mean "student". As there are several branches of Buddhism, it means different things in different branches.

http://en.wikipedia.org/wiki/Bodhisattva