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.
The “degree” (first, second, third cousin, etc.) indicates one less than the minimum number of generations between both cousins and the nearest common ancestor. For example, a person with whom one shares a grandparent (but not a parent) is a first cousin; someone with whom one shares a great-grandparent (but not a grandparent or a parent) is a second cousin; and someone with whom one shares a great-great-grandparent (but not a great-grandparent, grandparent or parent) is a third cousin; and so on.
The “removal” (once removed, twice removed, etc.) indicates the number of generations, if any, between the two cousins. The child of a first cousin is a “first cousin once removed” because the one generation gap represents one “removal”. The child is still considered first cousin, as the grandparent (the child’s great-grandparent), is the most recent common ancestor and thus represents one “degree”.
Non-genealogical usage often eliminates the degrees and removals, and refers to people with common ancestors merely as “cousins” or “distant cousins”. The system can handle kinships going back any number of generations (subject to the genealogical information being available).