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.

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.

People are related with a type of cousin relationship if they share a common ancestor, and are separated from their most recent common ancestor by two or more generations. This means neither person is an ancestor of the other, they do not share a parent (are not siblings), and neither is a sibling of the other's parent (are not the other's aunt/uncle nor niece/nephew). In the English system the cousin relationship is further detailed by the concepts of degree and removal.

The degree is the number of generations subsequent to the common ancestor before a parent of one of the cousins is found. This means the degree is the separation of the cousin from the common ancestor less one. Also, if the cousins are not separated from the common ancestor by the same number of generations, the cousin with the smallest separation is used to determine the degree. The removal is the difference between the number of generations from each cousin to the common ancestor. Two people can be removed but be around the same age due to differences in birth dates of parents, children, and other relevant ancestors.

In mathematics, cousin primes are prime numbers that differ by four. Compare this with twin primes, pairs of prime numbers that differ by two, and sexy primes, pairs of prime numbers that differ by six. The cousin primes below 100 are: (3, 7), (7, 11), (13, 17), (19, 23), (37, 41), (43, 47), (67, 71), (79, 83).

In mathematics, the Cousin problems were introduced in special cases by Pierre Cousin in 1895. They are now posed, and solved.

六级/考研单词: ancestor, kin, differentiate, usage, entity, illicit, medicare, niece, farther, mathematics, prime, differ, twin, pose