isomers

isomers (ī´sōmurz),

n.pl 1. organic compounds having the same empirical formula–i.e., the same number of the same atoms but different structural formulas and therefore different physical and chemical properties.
n 2. one of several nuclides having the same number of neutrons and protons but capable of existing, for a measurable time, in different quantum states with different energies and radioactive properties. The isomer of higher energy commonly decays to one with lower energy by a process known as isomeric transition.

isomers, optical,

n.pl two isomers whose structures, dextro- and levo-, differ only in a spatial arrangement that makes them mirror images. This occurs only when there is an asymmetric carbon atom–i.e., one attached to four different substituents. The pharmacologic activity often resides very largely in one of the two forms.

isomers, stereo-,

n.pl molecules that differ only in the spatial arrangement of the atoms. This term includes optical isomers.