The Collaborative International Dictionary of English v.0.48:

Formedon \For"me*don\ (f[^o]r"m[-e]*d[o^]n), n. [OF., fr. Latin.
   So called because the plaintiff claimed "by the form of the
   gift," L. per formam doni.] (O. Eng. Law)
   A writ of right for a tenant in tail in case of a
   discontinuance of the estate tail. This writ has been
   abolished.
   [1913 Webster]

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

FORMEDON, old English law. The writ of formedon is nearly obsolete, it
having been superseded by the writ of ejectment. Upon an alienation of the
tenant in tail, by which the estate in tail is discontinued, and the
remainder or reversion is by the failure, of the particular estate,
displaced and turned into a mere right, the remedy is by action of formedon,
(secundum formam doni,) because the writ comprehends the form of the gift.
This writ is in the nature of a writ of right, and the action of formedon is
the highest a tenant in tail can have. This writ is distinguished into three
species; a formedon in the descender, in the remainder, and in the reverter.
8 Bl. Com. 191 Bac. Ab. h.t.; 4 Mass. 64.