An idiom is defined as - a group of words in a fixed order that has a particular meaning that is different from the meanings of each word on its own.
on the tip of one's tongue
not quite able to remember something
The name of the movie is on the tip of my tongue.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.