The Collaborative International Dictionary of English v.0.48:

Affirmance \Af*firm"ance\, n. [Cf. OF. afermance.]
   1. Confirmation; ratification; confirmation of a voidable
      act.
      [1913 Webster]

            This statute . . . in affirmance of the common law.
                                                  --Bacon.
      [1913 Webster]

   2. A strong declaration; affirmation. --Cowper.
      [1913 Webster]

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

AFFIRMANCE. The confirmation of a voidable act; as, for example, when an
infant enters into a contract, which is not binding upon him, if, after
attaining his full age, he gives his affirmance to it, he will thereafter be
bound, as if it had been made when of full age. 10 N. H. Rep. 194.
     2. To be binding upon the infant, the affirmance must be made after
arriving of age, with a full knowledge that it would be void without such
confirmation. 11 S. & R. 305.
     3. An affirmance may be express, that is, where the party declares his
determination of fulfilling the contract; but a more acknowledgment is not
sufficient. Dudl. R, 203. Or it may be implied, as, for example, where an
infant mortgaged his land and, at full age, conveyed it, subject to the
mortgage. 15 Mass. 220. See 10 N. H. Rep. 561.