The Collaborative International Dictionary of English v.0.48:

Ex post facto \Ex" post` fac"to\, or Ex postfacto \Ex"
post`fac"to\ ([e^]ks" p[=o]st" f[a^]k"t[-o]). [L., from what is
   done afterwards.] (Law)
   From or by an after act, or thing done afterward; in
   consequence of a subsequent act; retrospective.

   Ex post facto law, a law which operates by after enactment.
      The phrase is popularly applied to any law, civil or
      criminal, which is enacted with a retrospective effect,
      and with intention to produce that effect; but in its true
      application, as employed in American law, it relates only
      to crimes, and signifies a law which retroacts, by way of
      criminal punishment, upon that which was not a crime
      before its passage, or which raises the grade of an
      offense, or renders an act punishable in a more severe
      manner that it was when committed. Ex post facto laws are
      held to be contrary to the fundamental principles of a
      free government, and the States are prohibited from
      passing such laws by the Constitution of the United
      States. --Burrill. --Kent.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ex post facto
    adj 1: affecting things past; "retroactive tax increase"; "an
           ex-post-facto law"; "retro pay" [syn: ex post facto,
           retroactive, retro]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "ex post facto":
   a priori, after, after all, after that, afterwards, back, backward,
   early, in the aftermath, in the sequel, into the past, later, next,
   retroactive, retrospective, since, subsequently, then, thereafter,
   thereon, thereupon, therewith



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

EX POST FACTO, contracts, crim. law. This is a technical expression, which
signifies, that something has been done after another thing, in relation to
the latter.
     2. An estate granted, may be made good or avoided by matter ex post
facto, when an election is given to the party to accept or not to accept. 1
Co. 146.
     3. The Constitution of the United States, art. 1, sec. 10, forbids the
states to pass any ex post facto law; which has been defined to be one which
renders the act punishable in a manner in which it was not punishable when
it was committed. 6 Cranch, 138. This definition extends to laws passed
after the act, and affecting a person by way of punishment of that act,
either in his person or estate. 3 Dall. 386; 1 Blackf. Ind. R. 193 2 Pet. U.
S. Rep. 413 1 Kent, Com. 408; Dane's Ab. Index, h.t.
     4. This prohibition in the constitution against passing ex post facto
law's, applies exclusively to criminal or penal cases, and not to civil
cases. Serg. Const. Law, 356. Vide 2 Pick. R. 172; 11 Pick. R. 28; 2 Root,
R. 350; 5 Monr. 133; 9 Mass. R. 363; 3 N. H. Rep. 475; 7 John. R. 488; 6
Binn. R. 271; 1 J. J. Marsh, 563; 2 Pet. R. 681; and the article
Retrospective.