What is an icon ? The word "icon" means "image," but since the early centuries of Christianity, the word "icon" is normally used to refer to images with a religious content, meaning and use.
Most icons are two-dimensional; mosaics, paintings, enamels, miniatures, but ancient three dimensional icons also exist. Many people assume an icon must be in a Byzantine or Russian style. Many icons are, but many are not; other Orthodox Christian cultures have their own traditional styles of art, and many icons exist painted in a Western style.
It is not style that makes a painting an icon, it is subject, meaning and use. An icon is always the representation of a religious subject, but not every representation of a religious subject is an icon. The militant atheism of the Communist regimes produced anti-Christian caricatures of religious themes; these are obviously not icons. Nor are sentimental or even erotic portraits of models or historical figures masquerading as images of the saints, and unfortunately such paintings are very common in Western Christian religious art. An icon is not simply the representation of a religious subject, it is a representation with a religious meaning, and if it is an Orthodox icon it must have an orthodox meaning.
It may seem surprising that an image can be unorthodox. But consider for a moment: an image represents something - or it misrepresents something, or perhaps it represents a mere fiction. An image can mislead and it can lie - or it can be inadequate. It is for this reason Orthodox tradition forbids certain kinds of religious image. The Synod in Trullo, for example, which was convened in 692 to complete the work of the Fifth and Sixth Oecumenical Councils, forbids the depiction of Christ as a lamb, despite this having been a common image in the past, and insists He be represented in His humanity. [Canon 82] The reason for this is clearly stated; the image is to lead us to remember "His life in the flesh, His Sufferings, His Saving Death, and the deliverance accomplished for the world." The Council of Moscow of 1667 forbade representations of God the Father as a human being - it was the Son Who took on humanity, not the Father, and representations of the Father in human form is deeply misleading.