From the Concise Oxford English Dictionary
One who violates his or her allegiance or acts disloyally to country, King, cause, religion, principle, themselves etc.
Theresa May’s misguided banning of Pamela Geller and Robert Spencer has become a worldwide news story. If the Home Secretary thought that this ban would quieten things down, then she will be very disapointed. Theresa May is a living, walking definition of the word ‘traitor’.