Amutation taking place in the body cells of an organism is called

Somatic mutation

Explanation:

Mutation is a spontaneous change in the DNA  of an organism. Somatic are cells that make up the body. A change in the DNA sequence in cells other than sperm or egg  is called somatic mutation.

The cells are highly replicative i.e., they are the cells responsible for growth of the general body organs. Somatic mutations can occur in any of the cells of the body except the germ cells (sperm and egg) and therefore are not passed on to children.