Who is a bramhin ?

No one is a Brahmin by birth.

At birth all are one and the same. The day 'upanayana' is conducted then he is known as 'dwijan'(twice born). By repeated chanting of the Gayathri when one earns 'tejas'(brilliance) he is known as 'Vipran'. He is known as 'Shastri' with the deep practice of the shastras. After learning the Vedas if he conducts yagna he is then known as 'Srowthi'. After being initiated into a 'Mantra' if he does japa (repeated chanting) of the 'mantra' and the darshan of the god or the goddess of the particular 'Mantra' is had, he is then known as 'Rishi'. He is known as 'Muni' if he controls his mind and does enquiry of the Self. If one attains oneness with the highest and sees only Hari everywhere and in everything he is then known as a 'Brahmin'.