Collateral consanguinity

Collateral consanguinity, sometimes called Germanic consanguinity, adopted by Pope Alexander II in the 11th century, changed this to defining the degree as the number of generations removed from the common ancestor (not counting the ancestor). Innocent III in 1215 restricted the impediment to the fourth degree, since tracing more distant ancestry was often difficult or impossible.

The first degree would include parents and children
First cousins would be within the second degree, as are uncle and niece
Second cousins would be within the third degree
Third cousins would be within the fourth degree

In the Roman system of calculating the degree of consanguinity, degrees are as follows:

The first degree of kinship includes parents and children (direct line)

The second degree of kinship includes
Brothers and sisters
Grandparents and grandchildren (direct line)

The third degree of kinship includes
Uncles or aunts and nieces and nephews
Great-grandchildren and great-grandparents (direct line)

The fourth degree of kinship includes
First cousins (children sharing a pair of common grandparents)
Great uncles or great aunts and grand nephews and grand nieces
Great grandchildren and great grandparents
.....
Roman civil law generally prohibited marriages within four degrees of consanguinity. Early Christian custom adopted some of these definitions and limits, though the extent of prohibition varied somewhat from culture to culture.

https://www.thoughtco.com/consanguinity-and-medieval-marriages-3529573

https://www.thoughtco.com/cousin-relationships-explained-3960560

Consanguinity and Medieval Marriages