Commonplace, platitude, truism, bromide, cliché mean an idea or expression lacking in originality or freshness.
A commonplace is a stock idea or expression which is frequently little more than the obvious, conventional, and easy thing to think or say on a given subject.
Platitude adds to commonplace the suggestions of flatness or triteness- and, often, utterance with an air of importance or novelty.
A truism is a self-evident truth; it differs from an axiom (see axiom at PRINCIPLE) in frequently implying a somewhat superfluous insistence upon the obvious.
Bromide applies to a commonplace, platitude, or truism that strikes the listener or reader as especially dull or hackneyed and, often, as an evidence of its maker’s lowgrade mentality.
Cliché applies to an expression which when new was fresh and full of meaning but which by constant iteration has become not only dull but hackneyed and stereotyped.