Expression is a noun that refers to the act of making your thoughts, feelings, etc. known by speech, writing, or some other method.
It can also mean:
1. A word or phrase
2. The look on someone's face that shows what they are thinking or feeling
3. In mathematics, a group of symbols that represent a quantity
4. In biology, the process by which information from a gene is used to make a functional product
5. In art, the quality of making your feelings or thoughts clear through works of art