What is the difference between a constant and a variable

A constant is a number that stays the same. A variable is number that doesn't remain the same.

A constant never changes. Its value is constant, which is a big part of the reason why it's called a "constant". " 42 " is constant. It doesn't depend on anything.

A variable can change. Its value varies, and that's why it's described as "variable". ' x ' is a variable. Its value depends on what ' x ' is.