illusion vs skewness

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. 

skewness

noun
  • The property of being skew. 

  • A measure of the asymmetry of the probability distribution of a real-valued random variable; is the third standardized moment, defined as scriptstyle 𝛾₁=(πœ‡β‚ƒ)/(𝜎³),! where πœ‡β‚ƒ is the third moment about the mean and 𝜎 is the standard deviation. 

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