Mathematical expressions are a combination of numbers, variables (like x, y, or z), and operators (like +, −, ×, and ÷) that represent a particular value or quantity. For example, 2+3 is a simple mathematical expression. The + is an operator that tells us to add the two numbers together.

The operation in the expression 30−12 is subtraction. When you subtract one number from another, you're essentially reducing the first number by the amount of the second number.

More Info: en.wikipedia.org