Question

Why does IUPAC recommend NOT to use the term geometrical /geometric isomerism?

Answer 1

The systematic approach followed for the nomenclature of organic compounds as recommended by the International Union of Pure and Applied Chemistry is referred to as IUPAC nomenclature of organic compounds (often abbreviated to IUPAC).

Complete answer:
Because optical isomers and rotational isomers are also geometric isomers, IUPAC advises against using the term "geometric isomerism" for cis/trans isomerism.
Geometric isomers, also known as stereoisomers, are substances with the same chemical and structural formulas but different spatial arrangements, according to IUPAC.
Geometric isomers include cis/trans isomers, optical isomers, and rotational isomers.
To prevent confusion, the terms cis/trans should only be used to describe stereoisomerism caused by restricted rotation in double bonds or rings.

Additional Information:
Due to the restricted rotation of double bonds, two compounds with the same molecular formula and structural formula differ in the spatial arrangement of atoms or groups of atoms. By this property we can identify compounds that show geometrical isomerism.

Note:
The IUPAC nomenclature initiative aims to establish a worldwide standard for naming substances in order to facilitate communication. The system's goal is to give each structure a distinct and clear name, as well as to compare each name to a unique and clear structure.