A number that when squared gives a negative result.
If you square a Real Number you always get a positive, or zero, result. For example 2×2=4, and (-2)×(-2)=4 as well.
So how can we square a number and get a negative result? Because we "imagine" that we can.
The "unit" imaginary numbers (the same as "1" for Real Numbers) is √(-1) (the square root of minus one), and its symbol is i, or j.
See: Real Number