To a muslim, the term ''unbeliever'' or 'non-believer'' refers to anyone who rejects Islam,its principles, and the belief that only muhammad's words are the accurate,true word of God.
Just as the Bible is interpreted differently by Christians of differing teachings(Roman Catholics and Jehovah's Witnesses,for example,do not share the same beliefs and practices),the koran is interpreted differently depending on which muslim you talk to as well. Some claim that the Koran is peaceful and instructs muslims to ''call non believers to Islam'', meaning that muslims should peacefully evangelize(much like the Jehovah's witnesses do when they go door to door). Yet other,more militant muslims believe that if non-believes fail to convert,then they must be fought or even killed. The most extreme of these Muslims are terrorists(a small percentage of all Muslims).

Sai Chi

Anybody that is not a Muslim is considered as an unbeleiver.