Thesaurus. The thesaurus of synonyms and related words is fully integrated into the dictionary. Click on the thesaurus category heading under the button in an entry to see the synonyms and related words for that meaning. must for anyone with an interest in the changing face of language. The Macmillan Dictionary blog explores English as it is spoken around the world today. Definition and synonyms of delusion from the online English dictionary from Macmillan Publishers Limited.