An icon (from Ancient Greek εἰκών (eikṓn) 'image, resemblance') is a religious work of art, most commonly a painting, in the cultures of the Eastern Orthodox, Oriental Orthodox, and Catholic churches. They are not simply artworks; "an icon is a sacred image used in religious devotion". The most common subjects include Christ, Mary, saints and angels. Although especially associated with portrait-style images concentrating on one or two main figures, the term also covers most of the religious images in a variety of artistic media produced by Eastern Christianity, including narrative scenes, usually from the Bible or the lives of saints.

Icons are most commonly painted on wood panels with egg tempera, but they may also be cast in metal or carved in stone or embroidered on cloth or done in mosaic or fresco work or printed on paper or metal, etc. Comparable images from Western Christianity may be classified as "icons", although "iconic" may also be used to describe the static style of a devotional image. In the Greek language, the term for icon painting uses the same word as for "writing", and Orthodox sources often translate it into English as icon writing.

Eastern Orthodox tradition holds that the production of Christian images dates back to the very early days of Christianity, and that it has been a continuous tradition since then. Modern academic art history considers that, while images may have existed earlier, the tradition can be traced back only as far as the 3rd century, and that the images which survive from Early Christian art often differ greatly from later ones. The icons of later centuries can be linked, often closely, to images from the 5th century onwards, though very few of these survive. Widespread destruction of images occurred during the Byzantine Iconoclasm of 726–842, although this did settle permanently the question of the appropriateness of images. Since then, icons have had a great continuity of style and subject, far greater than in the icons of the Western church. At the same time there have been change and development.