The Collaborative International Dictionary of English v.0.48:

Cognation \Cog*na"tion\, n. [L. cognatio.]
   1. Relationship by blood; descent from the same original;
      kindred.
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            As by our cognation to the body of the first Adam.
                                                  --Jer. Taylor.
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   2. Participation of the same nature. --Sir T. Browne.
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            A like temper and cognation.          --Sir K.
                                                  Digby.
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   3. (Law) That tie of consanguinity which exists between
      persons descended from the same mother; -- used in
      distinction from agnation.
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WordNet (r) 3.0 (2006):

cognation
    n 1: line of descent traced through the maternal side of the
         family [syn: matrilineage, enation, cognation]
    2: (anthropology) related by blood [syn: consanguinity, blood
       kinship, cognation] [ant: affinity]

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

COGNATION, civil law. Signifies generally the kindred which exists between
two persons who are united by ties of blood or family, or both.
     2. Cognation is of three kinds: natural, civil, or mixed. Natural
cognation is that which is alone formed by ties of blood; such is the
kindred of those who owe their origin to an illicit connexion, either in
relation to their ascendants or collaterals.
     3. Civil cognation is that which proceeds alone from the ties of
families as the kindred between the adopted father and the adopted child.
     4. Mixed cognation is that which unites at the same time the ties of
blood and family, as that which exists between brothers, the issue of the
same lawful marriage. 6; Dig. 38, 10.