imaginary
Pronunciation: /ɪˈmadʒɪn(ə)ri/
adjective
- 2 Mathematics (of a number or quantity) expressed in terms of the square root of a negative number (usually the square root of −1, represented by i or j). See also complex.

Origin:
late Middle English: from Latin imaginarius, from imago, imagin- 'image'

Spelling help
Remember that imaginary ends in -ary.