An idiom is a phrase or expression whose meaning is not predictable from the usual meanings of its constituent elements. Idioms are often specific to a particular language or culture and can sometimes be difficult for non-native speakers to understand because their meanings are figurative rather than literal.
Example: The idiom "kick the bucket" means to die. The expression does not relate to kicking or buckets in a literal sense; it is a figurative way of referring to death.
In a sentence: "After a long and fulfilling life, he finally kicked the bucket at the age of ninety."