An imaginary number is one that when squared gives a negative result.
Normally, with real numbers, when you square them, you always get a positive result.
With imaginary numbers, when you square them, the answer is negative. They are written like a real number, but with the letter i after them, like this:
The letter i is a number, which when multiplied by itself gives -1. This means that
This makes imaginary numbers very useful when we need to find the square root of a real negative number. For example we normally cannot find the square root of say –16. But using imaginary numbers we can: We understand this imaginary number result as "4 times the square root of negative one".
Remember: real and imaginary numbers are not "like" quantities. You cannot say, add a real to an imaginary. They are separate types of number.
If you pair a real number with an imaginary number, you get a thing called a complex number, which can be plotted on a two-dimensional plane. They look like this: See Complex Numbers for more.