\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.]
         After his foes' defeasance.           --Spenser.
2. A rendering null or void.
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.
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.