The answer is: "squares".
1. Therefore, the complete sentence is:
"A difference of two squares is an expression that contains two perfect squares, with one subtracted from the other".
2. It has the following form:
a²-b²
3. You can factor the expression above as below:
(a-b)(a+b)
4. For example:
x²-4 can be written as x²-2², then it can be factor as following:
x²-2²=(x-2)(x+2)