illusion vs optical illusion

illusion

noun
  • The state of being deceived or misled. 

  • A misapprehension; a belief in something that is in fact not true. 

  • Anything that seems to be something that it is not. 

  • A magician’s trick. 

optical illusion

noun
  • An image that is visually deceptive or misleading. 

How often have the words illusion and optical illusion occurred in a corpus of books? (source: Google Ngram Viewer )