The Collaborative International Dictionary of English v.0.48:

In posse \In` pos"se\ [L.]
   In possibility; possible, although not yet in existence or
   come to pass; -- contradistinguished from in esse.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Posse \Pos"se\, n.
   See Posse comitatus.
   [1913 Webster]

   In posse. See In posse in the Vocabulary.
      [1913 Webster]

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

IN POSSE. In possibility; not in actual existence; used in contradistinction
to in esse.