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In kinship terminology, a cousin is a relative with whom one shares one or more common ancestors. The term is rarely used when referring to a relative in one's immediate family where there is a more specific term to describe the relationship (e.g., one's parents, siblings and descendants). The term "blood relative" can be used synonymously and establishes the existence of a genetic link. A system of "degrees" and "removals" is used to describe the relationship between the two cousins and the ancestor they have in common. This system can handle kinships with common ancestors existing any number of generations in the past, though non-genealogical usage often eliminates the degrees and removals and refers to people with common ancestry as simply "cousins" or "distant cousin".

By extension, The term "cousin" can also be used when referring to the genetic relationships between humans and any other form of life, as per the theory of evolution of all life on Earth descending from one common ancestor. However the term in this sense is most commonly restricted to the fields of study surrounding ecological genetics