Is it religious art? Is it something meant to make church buildings look beautiful? Is it a visual aid, used for teaching?
In a sense ikons are all of these, but none of them captures the essence of what an ikon is.
Perhaps it is easier to see what an ikon is if we look at what an ikon is not.
Ad Orientem: What's wrong with this picture?
Yes. That's an Orthodox bishop in a Romanian Orthodox Church.
For the record, I have a rather high opinion of the late Pope. I am not unaware that he is widely regarded as a saint within the Roman Church. He may well be a saint. I don't decide those things. But I do know one thing. Neither he nor George H. W. Bush (a true gentleman and a better than average president), nor Mikhail Gorbachev were or are Orthodox. Two of the three depicted in iconography are still alive and one is I believe an avowed atheist!
Hat tip to Ken over at Hallowed Ground.
At least one can say that the figures in the pictures don't have haloes. That means they are not ikons, and one would not venerate them, even if they are in a church.
Here is a slightly different example. Some years ago a colleague of mine in the Missiology Department at the University of South Africa, Klippies Kritzinger, showed me a postcard with a picture of Steve Biko, painted in an icon style, and asked me, as an Orthodox Christian, what I thought of it. He also showed me a similar one of Gandhi. Both of them came from Bridge-Building Images, and there are several others available from that source (hat-tip to Book Reviews and More: Robert Lentz's Icons).
My answer was that I did not regard them as ikons. First because neither Steve Biko nor Gandhi were Orthodox Christians (Gandhi wasn't any kind of Christian). Secondly, because even if Steve Biko had been Orthodox, no Orthodox ikonographer would have painted him like that, with heavy prison bars in the background. If he had been Orthodox, and an ikon had been painted of him, then the prison bars would have been reduced, and he would probably have been showing either holding them, or with his hand resting upon them, to show that a martyr's death is a triumphant one.
This can be seen, for example, in ikons of St Catherine, who, it is said, was put to death on a wheel. She is shown with her hand resting on a wheel, not with the wheel dominating the background.
This is not to say that I don't think Gandhi and Steve Biko were good men. On the tomb of St Alphege of Canterbury, who was boned (rather than stoned) to death by heathen Danes, is the inscription "He who dies for truth and justice dies for Christ", and I think Gandhi and Steve Biko died for truth and justice, and the manner of Steve Biko's death was similar to that of St Alphege.
But that does not make the pictures of them Orthodox ikons.
My blogging friend Matt Stone writes quite a lot about Christian art, and gives many examples, including some examples of ikons. But many of the works he shows I wouldn't regard as ikons.
In my own writing I usually distinguish between "ikons" and "icons". Ikons with a "k" are Orthodox ikons, icons with c "c" are what you see and click on on your computer screen, or are celebs on steroids (often described as "iconic").
There is one more non-example of an ikon (or example of a non-ikon) that is perhaps worth mentioning. This one is supposed to be based on some famous painting, though I'm not sure which one. If anyone can tell me, I'll be grateful.
This too is not an ikon, but is rather a satire or parody of a religious picture, and is almost, in a sense a political cartoon. I'd say in belongs in the same genre as the pictures of the political figures in the Romanian Church, with the exception that those are admired, while this one is not.
If you'd like to know where we saw it, and how we came across it, you can read the story here.
See also Differences of Western religious art and Orthodox iconography.