In mathematics, a variable is a letter which can represent a variety of different numbers. For example: p = 2.2k . This can be used to convert a weight from kilograms (k) to pounds (p). Since you could substitute different numbers in for k it is called a variable.
The formula we just looked at is called an equation because it contains an equal sign with something on both sides.
A combination of variables and numbers using the operations of addition/subtraction, and multiplication/division without an equal sign or if there is an equal sign,without something on both sides of the equal sign, is known as an expression.
If an equation had a solution that means there is a number that could be substituted in place of the variable that would balance each side of the equation.