The Arabic word kafir (literally the one who "covers", usually translated as "disbeliever") is the Muslim term commonly translated into English as infidel. In the Islamic doctrinal sense, the term refers to a person who does not recognize the one God (Allah) - atheists and polytheists - and not to religions such as Christianity, Judaism - who are seen as "People of the Book (Ahl-e-kitab)".
Kafir, like infidel, has also come to be regarded as offensive, thus some Muslim scholars discourage its use due to the Quran's command to use kind words. It is even a punishable offense to use this term against a Jew or a Christian, under Islamic law. Muslims today however do tend to use the term indiscriminately in reference to all non-Muslims.