Commonly "cousin" refers to a "first cousin", a relative whose most recent common ancestor with the subject is a grandparent. More 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.
Degrees and removals are used to more precisely describe the relationship between cousins. Degree measures the separation, in generations, from the most recent common ancestor to one of the cousins (whichever is closest), while removal measures the difference in generations between the cousins themselves. To illustrate usage, a "second cousin" is a cousin with a degree of two. When the degree is not specified first cousin is assumed. A cousin that is "once removed" is a cousin with one removal. When the removal is not specified no removal is assumed. This definition distinguishes a cousin from an ancestor, descendant, sibling, aunt, uncle, niece, or nephew.
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 law, a first cousin is a type of third-degree relative.