In general meaning, images that portray a sacred entity and that are themselves regarded as sacred. In specific meaning, the term is typically used particularly for Christian images produced in the Eastern Rite churches, since the 6th century, according to prescribed formulas of subject (Christ, the Virgin, saints, and the great feasts) and composition, for devotional use in the church or home. Most commonly comprising tempera on panel, but they may be in any two-dimensional or relief medium, including fresco. Christians who venerate icons make the distinction between veneration and worship, although in actual practice the distinction is blurred. For a non-Christian image that is sacred or embodies a deity, use "idol" or "cult image."

Source: Art & Architecture Thesaurus

http://vocab.getty.edu/page/aat/300033945