In Islam specifically, tawhid (Arabic, "making one") refers to the belief that there is only one God. Its idolatrous counterpart is shirk ("association of others with God"). Tawhid is the first of Islam's six traditional articles of faith.
Denial of tawhid is Islam's only unforgivable sin, as revealed to Muhammad in Sura 4:116-121.
Sura 4:116: GOD does not forgive idol worship [maintained until death], and He forgives lesser offenses for whomever He wills. Anyone who idolizes any idol beside GOD has strayed far astray.