What is a common law trademark?

“Common law” refers to laws which come from court decisions, rather than laws that are created by statutes. A common law trademark is therefore a trademark which is protected by such court-created law, rather than a statute. Most states in the U.S. have statutes which allow for the registration of trademarks in that state, similar to the federal registration scheme. If someone uses a trademark in a state without registering pursuant to the state statutes, that trademark can still be protected pursuant to the provisions of the state’s common law, and this is known as a common law trademark.