The Collaborative International Dictionary of English v.0.48:

Aldehyde \Al"de*hyde\ ([a^]l"d[-e]*h[imac]d), n. [Abbrev. fr.
   alcohol dehydrogenatum, alcohol deprived of its hydrogen.]
   1. (Chem.) A colorless, mobile, and very volatile liquid
      obtained from alcohol by certain processes of oxidation.
      [1913 Webster]

   2. (Chem.) Any compound having the group -CHO. Methyl
      aldehyde, the simplest aldehyde, is more commonly called
      formaldehyde, H-CHO, and acetic aldehyde is now more
      commonly called acetaldehyde. The higher aldehydes may
      be solids. A reducing sugar typically contains the
      aldehyde group.
      [PJC]

   Note: The aldehydes are intermediate between the alcohols and
         acids, and differ from the alcohols in having two less
         hydrogen atoms in the molecule, as common aldehyde
         (called also acetaldehyde, acetic aldehyde or
         ethyl aldehyde), C2H4O; methyl aldehyde (called
         also formaldehyde), CH2O.
         [1913 Webster +PJC]

   Aldehyde ammonia (Chem.), a compound formed by the union of
      aldehyde with ammonia.
      [1913 Webster]

WordNet (r) 3.0 (2006):

aldehyde
    n 1: any of a class of highly reactive chemical compounds; used
         in making resins and dyes and organic acids