The Collaborative International Dictionary of English v.0.48:

Ad interim \Ad in"ter*im\[L.]
   Meanwhile; temporary.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ad interim
    adv 1: for an intervening time; temporarily; "elected to serve
           ad interim"

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

AD INTERIM. 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.