The Collaborative International Dictionary of English v.0.48:

Mammon \Mam"mon\ (m[a^]m"m[u^]n), n. [L. mammona, Gr. mammwna^s
   riches, Syr. mam[=o]n[=a]; cf. Heb. matm[=o]n a hiding place,
   subterranean storehouse, treasury, fr. t[=a]man to hide.]
   Riches; wealth; the god of riches; riches, personified.
   [1913 Webster]

         Ye can not serve God and Mammon.         --Matt. vi.
                                                  24.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mammon
    n 1: wealth regarded as an evil influence
    2: (New Testament) a personification of wealth and avarice as an
       evil spirit; "ye cannot serve God and Mammon"

The Devil's Dictionary (1881-1906):

MAMMON, n.  The god of the world's leading religion.  The chief temple
is in the holy city of New York.

    He swore that all other religions were gammon,
    And wore out his knees in the worship of Mammon.
                                                            Jared Oopf