Illustration, n.
Etymology: [L. illustratio: cf. F. illustration.]
1. The act of illustrating; the act of making clear and distinct; education; also, the state of being illustrated, or of being made clear and distinct.
2. That which illustrates; a comparison or example intended to make clear or apprehensible, or to remove obscurity.
3. A picture designed to decorate a volume or elucidate a literary work.
Shades of meaning
Fernald has no entry of its own for illustration; he lists it under allegory, sample.
Where he writes about it:
Any comparison, analogy, instance, example, tale, anecdote, or the like which serves to let in light upon a subject may be called an illustration, this word in its widest use including all the rest. — from allegory