Cousin

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" as the children of one's uncles and aunts.

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".

The ordinals in the terms "first cousins", "second cousins", "third cousins", refer to the number of generations to one's closest common ancestor. The number of "G" words used to describe this ancestor will determine how close the relationship is. For example, having "great-great-grandparents" in common would be third cousins.

When the cousins are not the same generation, they are described as "removed". In this case, the smaller number of generations to the common ancestor is used to determine the degree, and the difference in generations determines the number of times removed. Note that the ages of the cousins are irrelevant to the definition of the cousin relationship.

A child ("Sam") of one person ("Laura") and a child ("Ryan") of that one person's sibling ("Robert") are first cousins. First cousins share grandparents ("Joseph" and "Nancy").

Sam and Ryan are first cousins because they are non-siblings who share a pair of grandparents.

The children of first cousins share a second cousin relationship. Second cousins share great-grandparents. People occasionally mistake the child of their first cousin as their "second cousin" - however that would actually be a first cousin, once removed. The removal denotes the generational difference.

Sam and Susannah are second cousins because they are non-first cousins and non-siblings who share great-grandparents. In other words, Sam and Susannah's parents are cousins.

The grandchildren of two first cousins; also the children of two second cousins. Third cousins share great-great-grandparents.

Sam and Lyla are third cousins because they are non-second cousins, non-first cousins and non-siblings who share great-great-grandparents.

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Wikipedia