The Collaborative International Dictionary of English v.0.48:

In esse \In` es"se\ [L.]
   In being; actually existing; -- distinguished from in
   posse, or in potentia, which denote that a thing is not,
   but may be.
   [1913 Webster]

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

IN ESSE. In being. A thing in existence. It is used in opposition to in
posse. A child in ventre sa mere is a thing in posse; after he is born, he
is in esse. Vide 1 Supp. to Ves. jr. 466; 2 Suppl. to Ves. jr. 155, 191.
Vide Posse.