imaginary

Adjective

• not based on fact; unreal
  • the falsehood about some fanciful secret treaties- F.D.Roosevelt
    
  • a small child's imaginary friends
    
  • to create a notional world for oneself
    

Noun

• (mathematics) a number of the form a+bi where a and b are real numbers and i is the square root of -1

Related words

• Synonyms: 
  fanciful , notional , complex number , complex quantity , imaginary number