The Collaborative International Dictionary of English v.0.48:

Allah \Al"lah\, n. [ contr. fr. the article al the + ilah God.]
   The name of the Supreme Being, in use among the Arabs and the
   Mohammedans generally.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Allah
    n 1: Muslim name for the one and only God

The Devil's Dictionary (1881-1906):

ALLAH, n.  The Mahometan Supreme Being, as distinguished from the
Christian, Jewish, and so forth.

    Allah's good laws I faithfully have kept,
    And ever for the sins of man have wept;
        And sometimes kneeling in the temple I
    Have reverently crossed my hands and slept.
                                                         Junker Barlow