Most generally, in the lineal kinship system used in the English-speaking world, a cousin is a type of familial relationship in which two relatives are two or more familial generations away from their most recent common ancestor. Commonly, "cousin" refers to a first cousin – a relative of the same generation whose most recent common ancestor with the subject is a grandparent.

Degrees and removals are separate measures used to more precisely describe the relationship between cousins. Degree measures the separation, in generations, from the most recent common ancestor(s) to a parent of one of the cousins (whichever is closest), while removal measures the difference in generations between the cousins themselves, relative to their most recent common ancestor(s). To illustrate usage, a second cousin is a cousin with a degree of two; there are three (not two) generations from the common ancestor(s). When the degree is not specified, first cousin is assumed. A cousin "once removed" is a cousin with one removal. When the removal is not specified, no removal is assumed.

Various governmental entities have established systems for legal use that can precisely specify kinship with common ancestors any number of generations in the past; for example, in medicine and in law, a first cousin is a type of third-degree relative.