The opposing term, referring to the original entity after which something else was named, is called an eponym.
When namesake refers to something or someone who is named for something or someone else, the second recipient of a name is usually said to be the namesake of the first. This usage usually refers to humans named for other humans,   but current usage also allows things to be or have namesakes.   Sometimes the first recipient can also be called the namesake;  however, the correct and unambiguous term would be the eponym.
Naming a child after a relative, friend, or well-known person is a common practice in the English-speaking world. Continued practise of naming a child for the parent or grandparent may result in several relatives (e.g. cousins) being namesakes of each other despite not having been named for each other.
Among Ashkenazi Jews, it is customary to name a child after a dead relative, such as the child's grandparent, but never after a living person.  Sephardic Jews traditionally are encouraged to name their children after relatives, living or dead.  Greek families traditionally name a child after its paternal grandparents and the second child of the same sex is named after its maternal grandparents.
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When a son is named for his father, "Jr."/"II", "III'", or another name suffix may be added to the name of the son (and sometimes "Sr." or a prior number to the father's name), in order to distinguish between individuals, especially if both father and son become famous, as in the case of poet Oliver Wendell Holmes Sr. and his son, Oliver Wendell Holmes Jr., an Associate Justice of the United States Supreme Court.
Sometimes the "Jr." or "Sr." suffix is applied even when the child's legal name differs from that of the parent. One example is that of the singer Hiram King Williams, known professionally as Hank Williams, and his son Randall Hank Williams, known professionally as Hank Williams Jr. Daughters being named for their mothers using similar suffixes is less common. One example is thoroughbred jockey Rosemary Homeister Jr. whose mother was also a jockey before turning to train. A more archaic method of distinguishing father from son was to follow the name with the Elder or the Younger, respectively, for example William Pitt the Elder and William Pitt the Younger.
Buildings, such as the Trump Tower, and companies, like the Ford Motor Company, are often named for their founders or owners. Biologic species and celestial bodies are frequently named for their discoverers.  Alternatively, their discoverers may name them in honor of others.  Occasionally, material goods, such as toys or garments, may be named for people closely associated with them in the public mind. The teddy bear, for example, was named for President Theodore Roosevelt, because of a popular story in which the then-President objected to cruel treatment of a bear by hunters. 
The fedora hat may be considered the "namesake" of a fictional character, Princess Fédora Romanoff, from an 1887 play, Fédora, by Victorien Sardou. In her portrayal of that character, Sarah Bernhardt wore a soft felt hat with a center crease, which became known popularly as a "fedora". 
|Look up namesake in Wiktionary, the free dictionary.|
- Code name, word or name used clandestinely to refer to another name or word
- Cognomen, inherited name
- List of companies named after people
- Protected Geographical Status, product target name sourced to protected geographical name
- Scientific phenomena named after people
- "Namesake". Oxford Dictionaries. Oxford University Press. Retrieved 1 April 2019.
- "Namesake". Collins English Dictionary. HarperCollins. Retrieved 1 April 2019.
- "Namesake". Merriam-Webster Online Dictionary. Retrieved 1 April 2019.
- "Namesake". Dictionary.com Unabridged. Random House. Retrieved 1 April 2019.
- "Namesake". American Heritage Dictionary. Retrieved 1 April 2019.
- Harper, Douglas. "Namesake". Online Etymology Dictionary. Retrieved 14 March 2016.
- "The Laws of Jewish Names". Chabad.org. Chabad-Lubavitch Media Center. Retrieved 14 March 2016., citing Sefer Chassidim 460; Shaarei Halachah Uminhag, vol. 3, p. 298.
- See, e.g., Nowicke, Joan W. (September–October 1974). "Three New Species of Tournefortia (Boraginaceae) from the Andes and Comments on the Manuscripts of E. P. Killip". Bulletin of the Torrey Botanical Club. 101 (5): 229–234. doi: 10.2307/2484867. JSTOR 2484867. (species); and Committee on Small Body Nomenclature of Division III of the International Astronomical Union. "IAU Comet-naming Guidelines". IAU: Central Bureau for Astronomical Telegrams. IAU: Central Bureau for Astronomical Telegrams. Retrieved 14 March 2016. (comets).
- See, e.g., Platnick, Norman I. (10 June 1993). "A New Genus of the Spider Family Caponiidae (Araneae, Haplogynae) from California" (PDF). American Museum Novitates (3063): 1. Retrieved 14 March 2016. (species of spider named for actor Harrison Ford).
- "Teddy Bears". America's Story from America's Library. Library of Congress. Retrieved 14 March 2016.
- Harper, Douglas. "Fedora". Online Etymology Dictionary. Retrieved 14 March 2016.