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A somatic cell belongs to the body of a multicellular organism rather than its germline. Keratinocytes and cardiomyocytes are examples. The distinction concerns contribution to reproduction rather than a universal difference in chromosome number: many somatic cells are diploid, but specialized exceptions occur. A mutation in a somatic cell can spread through its descendants within the body without necessarily being inherited by offspring. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 20, Germ Cells and Fertilization: germ line and soma