stereoisomer

Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution), but that differ only in the three-dimensional orientations of their atoms in space.

e.g,

Stereoisomers can be classed as cis-trans isomers or optical isomers .

Cis-trans isomers - sometimes referred to as geometrical isomers - are stereoisomers. They occur when double bonds prevent rotation of atoms around a bond. For example cis-butenedioic acid and trans-butenedioic acid are stereoisomers. In the cis-isomer, both of the acid groups lie on the same side of the molecule. In the trans-isomer, the acid groups are on opposite sides of the molecule. 

cis and trans isomers of butenedioic acid

Optical isomers - also called enantiomers - are mirror images of one another. Just as a right-handed glove cannot be superimposed on a left-handed glove, optical isomers cannot be superimposed on one another. For example bromo-chloro-fluoro-methane exists in mirror image forms.