n.1 deception, delusion.
2 a misapprehension of the true state of affairs.
3 a the faulty perception of an external object. b an instance of this.
4 a figment of the imagination.
5 optical illusion.
Phrases and idioms:
be under the illusion (foll. by that + clause) believe mistakenly.
Derivatives:
illusional adj.
Etymology: ME f. F f. L illusio -onis f. illudere mock (as IN-(2), ludere lus- play)