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. be under the illusion (foll. by that + clause) believe mistakenly. illusional adj.
[ ME f. F f. L illusio -onis f. illudere mock (as IN-(2), ludere lus- play) ]