Learner's definition of VANGUARD
1 [count] : the group of people who are the leaders of an action or movement in society, politics, art, etc. — usually singular
a member of the feminist vanguard
They are at the vanguard of a revolution in medical research. [=they are leading a revolution in medical research]
She was in the vanguard of the feminist movement in the early seventies.
2 the vanguard : the soldiers, ships, etc., that are at the front of a fighting force that is moving forward