Bodhisattva (Skt., Tib. changchub sem pa): Somebody who strives for enlightenment for the benefit of all beings without ever losing courage.
This attitude corresponds to the ideal of the Great Way, to which the attitude in the Diamond Way also belongs.
On the one hand, a bodhisattva is somebody who has understood emptiness and has developed compassion; on the other hand, this term is used for those who have taken the Bodhisattva Promise.
The promise to accomplish buddhahood for the benefit of all beings and to work with diligence and strength until all beings are liberated or enlightened.
It is taken in the presence of a realized bodhisattva and is repeated in the context of daily meditation to strengthen this attitude.'
-- Quoted from 'Fearless Death' book by Lama Ole Nydahl.