Illusion, n.
Etymology: [F. illusion, L. illusio, fr. illudere, illusum, to illude. See Illude.]
1. An unreal image presented to the bodily or mental vision; a deceptive appearance; a false show; mockery; hallucination. To cheat the eye with blear illusions. Milton.
2. Hence: Anything agreeably fascinating and charning; enchantment; witchery; glamour. Ye soft illusions, dear deceits, arise! Pope.
3. (Physiol.)
A sensation originated by some external object, but so modified as in any way to lead to an erroneous perception; as when the rolling of a wagon is mistaken for thunder.
3 more senses
Note: Some modern writers distinguish between an illusion and hallucination, regarding the former as originating with some external object, and the latter as having no objective occasion whatever.
4. A plain, delicate lace, usually of silk, used for veils, scarfs, dresses, etc.
Syn. -- Delusion; mockery; deception; chimera; fallacy. See Delusion. Illusion, Delusion. Illusion refers particularly to errors of the sense; delusion to false hopes or deceptions of the mind. An optical deception is an illusion; a false opinion is a delusion. E. Edwards.
Shades of meaning
Fernald has no entry of its own for illusion; he lists it under delusion.
Where he writes about it:
A delusion is a mistaken conviction, an illusion a mistaken perception or inference. — from delusion
An illusion may be wholly of the senses; a delusion always involves some mental error. — from delusion
In an optical illusion the observer sees either what does not exist, or what exists otherwise than as he sees it, as when in a mirage distant springs and trees appear close at hand. — from delusion