Lewis & Short

illūsĭo (inl-), ōnis, f. [illudo],

  1. I. a mocking, jeering; irony, a figure of speech, = derisio, insultatio, Cic. de Or. 3, 53, 202; also ap. Quint. 9, 1, 28; cf. id. 8, 6, 54.
  2. II. An illusion, deceit (eccl. Lat.), Vulg. Psa. 37, 7; Isa. 66, 4.