If you've heard an expression a million times, chances are it's a cliche.
Cliche, also spelled cliché, is a 19th century borrowed word from the French which refers to a saying or expression that has been so overused that it has become boring and unoriginal. Think about the expressions "easy as pie," or "don't play with fire," or "beauty is skin deep." These are all cliches. A plot or action sequence in a film or novel can also be called a cliche if it has become dull and predictable through overuse.