What is the difference between “complain” and “complaint” (and also “to complain” and “to complaint”)?
“Complain” is a verb, an action that is done. Specifically, it the act of expressing dissent with the current or former state of something.
“Complaint” is a noun, an object that can act or be acted on(such as a person, a place, a thing or an idea). It means a “statement that a situation is unsatisfactory or unacceptable.” The term is often used in either a medical(to describe the ailment, injury, or symptoms one is suffering from) or legal(a formal accusation against either a person or state of affairs) context.
The two words are related in meaning; compare various words in English that act both as a verb and a noun, such as “access”, “answer”, and “claim”. Here is a list of such words.