a number that is expressed in terms of the square root of a negative number (usually the square root of −1, represented by i or j ).
This might explain it better than I can :
BTW....Isaac Asimov would argue with the term "Imaginary"....he claimed that they were just as "Real" as "Real" numbers !!!!!
......agree numbers that have an 'i' (as in IMAGINARY) component ...involves a sqrt of a negative number.... SOMETIMES '0' is considered imaginary too (at least that is what I found)