Geometrical isomerism is a type of stereoisomerism having the same molecular formula and same structure but differ in the relative arrangement of atoms.
An example of geometric isomerism you will meet at an introductory level is but-2-ene. In one case, the CH3 groups are on opposite sides of the double bond, and in the other case, they are on the same side. The two most common types of geometric isomers are those arising from a double bond and those arising from a ring structure. These type of geometric isomers are also called cis/trans isomers. When two identical groups occupy adjacent positions, the isomer is called cis and when arranged opposite to one another, the isomer is called trans. e.g