Which of the following best describes what constitutes a trademark?

A trademark is primarily defined as a symbol, word, or device that identifies and distinguishes goods or services of one source from those of others. This key aspect is crucial because trademarks serve to protect consumers by preventing confusion about the origin of products and services. When a trademark is registered, it provides legal recognition and protection to the owner, allowing them exclusive rights to use that mark in commerce.

The other options do not accurately capture the essence of what a trademark is. While a design and functionality can sometimes be part of a trademark, they do not define it. A unique song may have trademark potential depending on its use, but it is not exclusively a trademark. Lastly, licensing agreements relate to the use of trademarks but do not constitute the trademark itself; rather, they are contractual arrangements regarding the rights to use the trademark. Thus, the defining characteristic of a trademark is its ability to indicate the source of goods or services, which is encapsulated in the description of a word, logo, or device.