What is Actus Inceptus, Cujus Perfectio Pendet Ex Voluntate Partium, Revocari Potest; Si Autem Pendet Ex Voluntate Tertiae Personae, Vel Ex Contingenti, Revocari Non Potest?

Legal Definition
An act already begun, the completion of which depends on the will of the parties, may be revoked; but if it depend on the will of a third person or on a contingency, it cannot be revoked. Bac. Max. reg. 20.
-- Black's Law Dictionary
Legal Definition
An act the completion of which depends upon the will of the parties may be revoked, but if it depends upon the will of third parties or upon a contingency, it cannot be revoked.
-- Ballentine's Law Dictionary