The Collaborative International Dictionary of English v.0.48:

highchair \high"chair`\, high chair \high" chair`\n.
   a chair designed for feeding a very young child, having four
   long legs and a footrest and a detachable tray, which rests
   in front of the child, holds the food, and also serves as a
   restraint, to keep the child from falling out of the chair.

   Syn: feeding chair.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

highchair
    n 1: a chair for feeding a very young child; has four long legs
         and a footrest and a detachable tray [syn: highchair,
         feeding chair]