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The Collaborative International Dictionary of English v.0.48 (gcide)

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]