Learner's definition of TRAITOR
: a person who is not loyal to his or her own country, friends, etc. : a person who betrays a country or group of people by helping or supporting an enemy
She has been called a traitor to the liberal party's cause.
He was a traitor who betrayed his country by selling military secrets to the enemy.
He turned traitor. [=he became a traitor; he betrayed his country/friends]
— traitorous/ˈtreɪtərəs/adjective,[more traitorous; most traitorous]