Cousin, once removed

A cousin is a relative with whom a person shares one or more common ancestors. In the general sense, cousins are two or more generations away from any common ancestor, thus distinguishing a cousin from an ancestor, descendant, sibling, aunt, uncle, niece, or nephew. However in common parlance "cousins" typically refers to "first cousins", individuals who share a grandparent.

Systems of "degrees" and "removals" are used in the English-speaking world to describe the exact relationship between two cousins (in the broad sense) and the ancestor they have in common. Various governmental entities have established systems for legal use that can precisely specify kinships with common ancestors existing any number of generations in the past, though common usage often eliminates the degrees and removals and refers to people with common ancestry as simply "distant cousins" or "relatives".

People are related with a type of cousin relationship if they share a common ancestor and the most recent common ancestor is more than two generations apart for both people. This means neither person is an ancestor of the other (descendants and ancestors), they do not share a parent (siblings), and neither is a sibling of a common ancestor (aunts/uncles and nieces/nephews).

The cousin relationship is further detailed by degree and removal. For example the second cousin once removed relationship is a second degree cousin with one removal.

The removal of the cousin relationship is the number of generations the cousins are apart. When the cousins are separated by a different number of generations from the most recent common ancestor, the cousin relationship is removed. The difference between the number of generations for each cousin is the removal. For example if the most recent common ancestor is 2 generations prior for one person and 3 generations prior for the other (one person's grandfather is the other person's great-grandfather) or the most recent common ancestor is 3 generations prior for one person and 4 generations prior for the other (one person's great-grandfather is the other person's great-great-grandfather) the cousins are separated by one generation and therefore once removed. Note that two people can be removed but be around the same age due to differences in birth dates of parents children and other relevant ancestors.

The degree of the cousin relationship is the number of generations prior to the parents before a most recent common ancestor is found. If the cousins are removed, the smaller number of generations to the most recent common ancestor is used to determine the degree of the cousin relationship. For example if one of the cousins has to go back one generation beyond their parents (the grandparents) before finding the most recent common ancestor and the other has to go back one or more they are first cousins. If one had to go back two generations beyond the parents (great grandparents) and the other had to go back two or more they would be second cousins.

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Wikipedia