The Collaborative International Dictionary of English v.0.48:

Defeasance \De*fea"sance\, n. [OF. defesance, fr. defesant, F.
   d['e]faisant, p. pr. of defaire, F. d['e]faire, to undo. See
   Defeat.]
   1. A defeat; an overthrow. [Obs.]
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            After his foes' defeasance.           --Spenser.
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   2. A rendering null or void.
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   3. (Law) A condition, relating to a deed, which being
      performed, the deed is defeated or rendered void; or a
      collateral deed, made at the same time with a feoffment,
      or other conveyance, containing conditions, on the
      performance of which the estate then created may be
      defeated.
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   Note: Mortgages were usually made in this manner in former
         times, but the modern practice is to include the
         conveyance and the defeasance in the same deed.
         [1913 Webster]

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

DEFEASANCE, contracts, conveyancing. An instrument which defeats the force
or operation of some other deed or estate. That, which in the same deed is
called a condition, in another deed is a defeasance.
     2. Every defeasance must contain proper words, as that the thing shall
be void. 2 Salk. 575 Willes, 108; and vide Carth. 64. A defeasance must be
made in eodem modo, and by, matter as high as the thing to be defeated; so
that if one be by deed) the other must also be by deed. Touchs. 397.
     3. It is a general rule, that the defeasance shall be a part, of the
same transaction with the conveyance; though the defeasance may be dated
after the deed. 12 Mass. R. 13 Pie P. 413 1 N. 11. Rep. 41; but see 4 Yerg.
57, contra. Vide Bouv. Inst. Index, h.t.; Vin. Ab. h.t.; Com. Dig. h.t.;
Id. Pleader, 2 W 35, 2 W 37; Lilly's Reg. h.t.; Nels. Ab. h.t.; 2 Saund.
47 n, note 1; Cruise, Dig. tit. 32, c. 7,, s. 25; 18 John. R. 45; 9 Wend. R.
538; 2 Mass. R. 493.