Germline mutation: A heritable change in the DNA that occurred in a germ cell (a cell destined to become an egg or in the sperm) or the zygote (the conceptus) at the single-cell stage. When transmitted to a child, a germline mutation is incorporated in every cell of their body.
Germline mutations play a key role in genetic diseases. They play a role, too, in certain types of cancer as, for example, the eye tumor retinoblastoma and Wilms tumor, a childhood malignancy of the kidney.
A germline mutation is in contrast to a somatic mutation which is acquired in a single body cell.