Geometric isomerism occurs when molecules have the same chemical formula but different shapes.

Geometric isomerism, also known as cis-trans isomerism, occurs when molecules have the same chemical formula but different shapes. This phenomenon can be explained using the VSEPR theory, which states that all atoms in a molecule are surrounded by a cloud of electrons. The shape of this cloud depends on the arrangement of bonds between the atoms and determines how large or small the molecule is. Molecules with different arrangements can therefore have very similar structures, even though they may be made up of entirely different atoms.

This video from CGP Grey explains how geometric isomerism works in more detail.