The definition of an adage is a saying that has come to be accepted as truth over time.(noun)
"The grass is always greener on the other side" is an example of an adage.
See adage in Webster's New World College Dictionary
Origin: Fr < L adagium, adagio < ad-, to + aio, I say < *agyo < IE base *ē-, speak, say
See adage in American Heritage Dictionary 4
Learn more about adage