Merriam-Webster's Word of the Day for August 21, 2016 is:
hypocorism \hye-PAH-kuh-riz-um\ noun
1 : a pet name
2 : the use of pet names
People began to refer to the elusive and mysterious Loch Ness monster by the hypocorism "Nessie" in the 1940s.
"… the use of hypocorisms … is on the decline (only my Aunt Dorothy is still called Toots), and terms of endearment have come under suspicion ('Call me Dollboat or Sweetie-Pie one more time, Mr. Snodgrass, and you've got a harassment suit on your hands')." — William Safire, The New York Times, 27 Sept. 1992
Did you know?
In Late Latin and Greek, the words hypocorisma and hypokorisma had the same meaning as hypocorism does in English today. They in turn evolved from the Greek verb hypokorizesthai ("to call by pet names"), which itself comes from korizesthai ("to caress"). Hypocorism joined the English language in the mid-19th century and was once briefly a buzzword among linguists, who used it rather broadly to mean "adult baby talk"—that is, the altered speech adults use when supposedly imitating babies. Once the baby talk issue faded, hypocorism settled back into being just a fancy word for a pet name. Pet names can be diminutives like "Johnny" for "John," endearing terms such as "honey-bunch," or, yes, names from baby talk, like "Nana" for "Grandma."