Second cousins

Commonly "cousin" refers to a "first cousin". First cousins are individuals who's most recent common ancestor is their grandparent.

More generally a cousin is a type of familial relationship where people with a common ancestor are two or more generations away from the most recent common ancestor. This distinguishes a cousin from an ancestor, descendant, sibling, aunt, uncle, niece, or nephew.

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.

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Wikipedia