A term, sometimes
derogatory, used by
practising (or, according to some,
fundamentalist)
Muslims to describe someone who is
Muslim by birth or name, but does not perform all of the religious duties expected of a Muslim.
Whether someone is a "real" or nominal Muslim is not a matter that can generally be known, since faith is in the heart, but there are indications. If it is clear that a person calling themselves a Muslim does not complete the Five Pillars of Islam, or rejects the need to perform them, then this is a pretty strong indication that they are a nominal Muslim.