The Collaborative International Dictionary of English v.0.48:

Ambulance \Am"bu*lance\, n. [F. ambulance, h[^o]pital ambulant,
   fr. L. ambulare to walk. See Amble.] (Mil.)
   (a) A field hospital, so organized as to follow an army in
       its movements, and intended to succor the wounded as soon
       as possible. Often used adjectively; as, an ambulance
       wagon; ambulance stretcher; ambulance corps.
   (b) An ambulance wagon or cart for conveying the wounded from
       the field, or to a hospital.
       [1913 Webster]

WordNet (r) 3.0 (2006):

ambulance
    n 1: a vehicle that takes people to and from hospitals