member of a group of genes that interact additively to influence a phenotypic trait

Polygenic is a common adjective in genetics. It means "many genes". It is used when a genetic character (phenotypic trait) depends on more than one gene.

It is very common and normal for a person's weight, for example, to depend on many causes, both genetic (inherited) and non-genetic (e.g. way of life). Here the use of polygenic means that various genes influence the weight of a person, as well as the non-genetic factors (life-style). The branch of genetics which studies polygenes is called quantitative genetics.