It can be used in various ways:
* A thing that completes or makes something else perfect: "The flowers were a perfect complement to the table setting."
* The act of completing or making something else perfect: "The addition of a new wing was the final complement to the hospital's expansion."
* A word or phrase that completes the meaning of a verb, adjective, or other word: "In the phrase 'She is beautiful,' 'beautiful' is a complement to the verb 'is'."
It's important to note that "complement" is both a noun and a verb.