expression
In French, 'expression' is used in a very similar context as it is in English, referring to a particular word, phrase or form of words that you use to express an idea or belief. It can also refer to the process of making known one's thoughts or feelings.
It's a common expression in French
This sentence explains that the expression being referred to is frequently used in the French language.
The expression of his feelings was sincere
This sentence is about a person who is expressing his feelings honestly and without any pretense.
I don't understand this expression
This sentence indicates that the speaker does not comprehend the meaning of a certain expression.