Geometrical Isomerism:           

Isomers are two molecules with the same molecular formula, but differ in the way the atoms are arranged.  Geometrical isomers are one form of stereoisomers which have identical molecular formulae, the atoms are bonded together in the same order but the arrangement of atoms in space are different.  Optical isomers are another form of stereoisomers.  Geometrical isomerism is also called cis-trans isomerism.

Cis-trans isomers occur in organic compounds which contain a carbon-carbon double or triple bond.  An example is in but-2-ene.

The cis form is where the substituent groups are on the same side of the double bond.  The trans form is where they on opposite sides.