The Collaborative International Dictionary of English v.0.48:

Agnation \Ag*na"tion\, n. [L. agnatio: cf. F. agnation.]
   1. (Civil Law) Consanguinity by a line of males only, as
      distinguished from cognation. --Bouvier.
      [1913 Webster]

WordNet (r) 3.0 (2006):

agnation
    n 1: line of descent traced through the paternal side of the
         family [syn: patrilineage, agnation]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

AGNATION, 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.