The Collaborative International Dictionary of English v.0.48:

Isomer \I"so*mer\ ([imac]"s[-o]*m[~e]r), n. [See Isomeric.]
   (Chem.)
   A compound which is isomeric with another body or compound; a
   compound having the same chemical composition as another
   compound; a member of an isomeric series.
   [1913 Webster +PJC]

   Note: An isomer may be a structural isomer, in which some
         of the atoms are bonded to different atoms (as in
         CH3.CH2.NH2 and CH3.NH.CH3), or an optical
         isomer, in which the number and types of chemical
         bonds are identical, but the arrangement of the atoms
         in three-dimensional space is different; the latter are
         also called stereoisomers.
         [PJC]