An idiom is a group of words, or in other words, a phrase that has a meaning other than its literal meaning of the words in it.
Idiom:
upset the applecart
Meaning:
to ruin or spoil a plan or idea
Example:
"Try not to upset the applecart as we have spent a lot of time working on this project."
List of common idioms, their meanings and examples of their usage in sentences
An expression in the usage of a language that is peculiar to itself either in having a meaning that cannot be derived from the conjoined meanings of its elements or in its grammatically atypical use of words.