The Collaborative International Dictionary of English v.0.48:

Provost \Prov"ost\, n. [OF. provost (L. prae and pro being
   confused), F. prev[^o]t, fr. L. praepositus placed before, a
   chief, fr. praeponere to place before: cf. AS. pr[=a]fost,
   pr[=o]fast. See Preposition, and cf. Propound.]
   1. A person who is appointed to superintend, or preside over,
      something; the chief magistrate in some cities and towns;
      as, the provost of Edinburgh or of Glasgow, answering to
      the mayor of other cities; the provost of a college,
      answering to president; the provost or head of certain
      collegiate churches.
      [1913 Webster]

   2. The keeper of a prison. [Obs.] --Shak.
      [1913 Webster]

   Note: In France, formerly, a provost was an inferior judge
         who had cognizance of civil causes. The grand provost
         of France, or of the household, had jurisdiction in the
         king's house, and over its officers.
         [1913 Webster]

   Provost marshal (often pronounced ?).
      (a) (Mil.) An officer appointed in every army, in the
          field, to secure the prisoners confined on charges of
          a general nature. He also performs such other duties
          pertaining to police and discipline as the regulations
          of the service or the commander's orders impose upon
          him.
      (b) (Nav.) An officer who has charge of prisoners on trial
          by court-martial, serves notices to witnesses, etc.
          [1913 Webster]

WordNet (r) 3.0 (2006):

provost marshal
    n 1: the supervisor of the military police