The coin mixer is covered in full in the entry Crypto Mixer or Crypto Tumbler, which explains how mixing works, its legal status and its links to money laundering. This page gives a short definition and explains the different names.
A coin mixer is a service that takes cryptocurrency from many users, pools it and sends out equivalent amounts to new addresses, so that observers of the public blockchain cannot easily tell which incoming coins match which outgoing ones. Centralized mixers hold users' funds during the process, while smart-contract mixers use code and cryptography so that no operator takes custody. Mixing can protect legitimate financial privacy, but it is also used to launder stolen or illicit funds, which is why regulators and law enforcement have sanctioned or prosecuted several mixing services and why exchanges treat mixed coins as high risk under anti-money laundering rules. A related, operator-free technique is CoinJoin, and the chaining of several services is described under daisy chaining tumblers.
"Coin mixer", "crypto mixer", "bitcoin mixer" and "tumbler" are used interchangeably; "tumbler" comes from the idea of tumbling coins together. All describe the same service, discussed further under cryptocurrency money laundering.