Definition of Bitcoin mixing:
A cryptocurrency tumbler or cryptocurrency mixing service[1] is a service that mixes potentially identifiable or "tainted" cryptocurrency funds with others, so as to obscure the trail back to the fund's original source.
Even this definition of Bitcoin mixing by Wikipedia is a bit questionable. The connotation is still almost referring to money laundering, which isn't really true. Bitcoin mixing services in this definition simply doesn't appear as a neutral tool for privacy, which is wrong. It seems it is something that is not normally used by ordinary people.
If I were to define a Bitcoin mixing service, I'd simply say it's a tool by which Bitcoin funds are mixed with others to protect the user's privacy.