The Collaborative International Dictionary of English v.0.48:

Underwear \Un"der*wear`\, n.
   That which is worn under the outside clothing; underclothes.
   [1913 Webster]

WordNet (r) 3.0 (2006):

underwear
    n 1: undergarment worn next to the skin and under the outer
         garments [syn: underwear, underclothes,
         underclothing]