The Collaborative International Dictionary of English v.0.48:

Vanguard \Van"guard`\, n. [For vantguard, avantguard, F.
   avant-garde; avant before, fore + garde guard. See Avant,
   Ab-,Ante-, and Guard, and cf. Advance, Vamp, Van
   of an army, Vaward.] (Mil.)
   The troops who march in front of an army; the advance guard;
   the van.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vanguard
    n 1: the leading units moving at the head of an army [syn:
         vanguard, van]
    2: any creative group active in the innovation and application
       of new concepts and techniques in a given field (especially
       in the arts) [syn: avant-garde, vanguard, van, new
       wave]
    3: the position of greatest importance or advancement; the
       leading position in any movement or field; "the Cotswolds
       were once at the forefront of woollen manufacturing in
       England"; "the idea of motion was always to the forefront of
       his mind and central to his philosophy" [syn: vanguard,
       forefront, cutting edge]