What is Agnation?

Legal Definition
In descents. The relation by blood which exists between such males as are descended from the same father; in distinction from cognation or consanguinity, which includes the descendants from females. This term is principally used in the civil law.
-- Bouviers Law Dictionary
Legal Definition
Kinship by the father's side. See Agnates; Agnati.
-- Black's Law Dictionary
Legal Definition
Relationship through males.
-- Ballentine's Law Dictionary