Definition from Wiktionary, the free dictionary
vanguard (plural vanguards)
- The leading units at the front of an army or fleet.
- (by extension) The person(s) at the forefront of any group or movement.
1922, Ben Travers, chapter 4, A Cuckoo in the Nest:
- By some paradoxical evolution rancour and intolerance have been established in the vanguard of primitive Christianity. Mrs. Spoker, in common with many of the stricter disciples of righteousness, was as inclement in demeanour as she was cadaverous in aspect.
leading units of an army
forefront of any group or movement