The Collaborative International Dictionary of English v.0.48:

In rem \In rem\ [L.] (Law)
   Lit., in or against a (or the) thing;

   Note: used:
   (a) Of any right (called

   right in rem or

   jus in rem) of such a nature as to be available over its
      subject without reference to one person more than another,
      or, as generally expressed, a right competent, or
      available, against all persons. Rights in rem include not
      alone rights over physical property, but all rights
      available against all persons indifferently, as those of
      life, liberty, and reputation.
   (b) Of actions for recovering or reducing to possession or
       enjoyment a specific object, as in the enforcement of
       maritime liens against a vessel, which is made the
       defendant by a sort of personification. Most actions for
       the specific recovery of property in English and American
       law are in the nature of actions in personam against a
       person alleged to be unlawfully withholding the property.
       [Webster 1913 Suppl.]

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

IN REM, remedies. This technical term is used to designate proceedings or
actions instituted against the thing, in contradistinction to personal
actions which are said to be in personam. Proceedings in rem include not
only judgments of property as forfeited, or as prize in the admiralty, or
the English exchequer, but also the decisions of other courts upon the
personal status, or relations of the party, such as marriage, divorce,
bastardy, settlement, or the like. 1 Greenl. Ev. Sec. 525, 541.
     2. Courts of admiralty enforce the performance of a contract by seizing
into their custody the very subject of hypothecation; for in these case's
the parties are not personally bound, and the proceedings are confined to
the thing in specie. Bro. Civ. and Adm. Law, 98; and see 2 Gall. R. 200; 3
T. R. 269, 270.
     3. There are cases, however, where the remedy is either in personam or
in rem. Seamen, for example, may proceed against the ship or cargo for their
wages, and this is the most expeditious mode; or they may proceed against
the master or owners. 4 Burr. 1944; 2 Bro. C. & A. Law, 396. Vide,
generally, 1 Phil. Ev. 254; 1 Stark. Ev. 228; Dane's Ab. h.t.; Serg. Const.
Law, 202, 203, 212.