ˈfænsɪfəl ɪˈmædʒɪn(ə)ɹi ɪˈmædʒɪˌnɛɹi ɪˈmædʒɪn(ə)ɹi ɪˈmædʒɪˌnɛɹi ˈnəʊʃənəl not based on fact; existing only in the imagination the falsehood about some fanciful secret treaties a small child's imaginary friends to create a notional world for oneself (mathematics) a number of the form a+bi where a and b are real numbers and i is the square root of -1