The Collaborative International Dictionary of English v.0.48:

Mohammedan \Mo*ham"med*an\, a. [From Mohammed, fr. Ar.
   muh['a]mmad praiseworthy, highly praised.]
   Of or pertaining to Mohammed, or the religion and
   institutions founded by Mohammed; in the latter sense,
   synonymous with Islamic, the term preferred by Moslems.
   [Written also Mahometan, Mahomedan, Muhammadan, etc.]
   [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Mohammedan \Mo*ham"med*an\, n.
   A follower of Mohammed, the founder of Islam (also called
   Islamism or Mohammedanism); an adherent of Islam; one who
   professes Mohammedanism or Islamism; a Muslim; a Moslem; a
   Musselman; -- this term is used mostly by non-Moslems, and
   some Moslems find it offensive. [Written also Muhammadan,
   Mahometan, Mahomedan, etc.]
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

Mohammedan
    adj 1: of or relating to the Arabian prophet Muhammad or to the
           religion he founded [syn: Muhammadan, Mohammedan]
    n 1: a follower of Mohammed [syn: Mohammedan, Muhammedan,
         Muhammadan]