Which of the following provides a clear definition of a trademark?

A trademark is defined as a symbol, word, or phrase that is legally established through use in commerce to identify and distinguish the source of goods or services of one party from those of others. This definition emphasizes the legal recognition of trademarks, which arises from their use in the marketplace rather than simply through registration. Trademarks serve as identifiers that inform consumers about the quality and origin of products, thereby providing protection for both businesses and consumers.

The other options do not pertain to the definition of a trademark. A category of patent protection addresses inventions and innovations, a general term for public domain works relates to materials free for use without permission, and a type of license for educational purposes deals with permissions under specific conditions. Thus, none of these alternatives capture the essence or legal nature of a trademark as effectively as the correct choice does.