California Code, Probate Code - PROB § 13

(a) The degree of kinship or consanguinity between two persons is determined by counting the number of generations separating those persons, pursuant to subdivision (b) or (c).  Each generation is called a degree.

(b) Lineal kinship or consanguinity is the relationship between two persons, one of whom is a direct descendant of the other.  The degree of kinship between those persons is determined by counting the generations separating the first person from the second person.  In counting the generations, the first person is excluded and the second person is included.  For example, parent and child are related in the first degree of lineal kinship or consanguinity, grandchild and grandparent are related in the second degree, and great-grandchild and great-grandparent are related in the third degree.

(c) Collateral kinship or consanguinity is the relationship between two people who spring from a common ancestor, but neither person is the direct descendent of the other.  The degree of kinship is determined by counting the generations from the first person up to the common ancestor and from the common ancestor down to the second person.  In counting the generations, the first person is excluded, the second person is included, and the common ancestor is counted only once.  For example, siblings are related in the second degree of collateral kinship or consanguinity, an aunt or uncle and a niece or nephew are related in the third degree, and first cousins are related in the fourth degree.