An eponym is someone or something whose name is or is thought to be the source of something's name (such as a city, country, era, or product); alternately it can be used to refer to the name of something that is based on or derived from someone or something else's name. Albert Einstein is the eponym of the element einsteinium; conversely, einsteinium is an eponym of Albert Einstein.

There are many different types of eponyms, especially in scientific fields. Theories, laws, equations, proofs, and elements often have their eponyms in the people that first discovered or proved them.

Proprietary eponyms are another matter entirely. These are general words that are, or were at one time, proprietary brand names or service marks. Kleenex, for example, is a brand of facial tissues, yet the word is used today to refer to facial tissues of any brand. Xerox is a brand of photocopy machine; that word, too, has been since adopted to refer to any brand of photocopy machine and, moreover, also employed as a verb to describe the act of photocopying. As this illustrates, although brand names are proper adjectives (as in, "Kleenex facial tissues"), when such terms are adopted for general use they tend to become nouns and often also verbs.

Some proprietary eponyms are given below. Where two spellings are given, the first spelling is the version in common use, while the second, parenthesized spelling refers to the original product name or service mark.