Jain is a believer or follower of Jainism Religion.
"Jain" or more properly speaking "Jaina" means a follower of Jina, which is applied to those persons who have conquered the lower nature, passion, hatred, etc. The word ‘Jain’ comes from the word ‘Jina’ which means a conqueror. ‘Jina’ comes from the root ‘Ji’—’to conquer’. It means conquering the passions. It does not mean conquering nations. The passions are considered as enemies of the soul. They taint the natural qualities of the soul, obscure right belief, cause false knowledge and wrong conduct. Lust, anger, pride and greed are considered as the major passions.
Follower of Christianity is called a Christian, Follower of Hinduism is called Hindu similarly follower of Jainism is called Jain.