What is Ad Interim?

Legal Definition
The Latin phrase ad interim (abbr. ad int., literally "in the time between") means "in the meantime" or "temporarily".

A diplomatic officer who acts in place of an ambassador is called chargé d'affaires ad interim.
-- Wikipedia
Legal Definition
In the mean time. An officer is sometimes appointed ad interim, when the principal officer is absent, or for some cause incapable of acting for the time. AD LARGUM. At large; as, title at large, assize at large. See Dane's Abr. ch. 144,
-- Bouviers Law Dictionary
Legal Definition
In the mean time. An officer ad interim is one appointed to fill a temporary vacancy or to discharge the duties of the office during the absence or temporary incapacity of its regular incumbent.
-- Black's Law Dictionary
Legal Definition
In the meantime.
-- Ballentine's Law Dictionary