I got hung up on a matter of terminology while revising a section about Spanish nicknames in my book: are nicknames like Mabel for María Ísabel acronyms? Not according to the English definition: “an abbreviation formed from the initial letters of other words and pronounced as a word.” However, the RAE’s Ortografía, my source for the Mabel example, states that it is an acronym (p. 628). When I looked up the RAE’s own definition of acronyms, copied below, I saw that the first meaning matches the English definition: an acrónimo is a sigla, or initialism. The second meaning, though, is broader: “A word formed by the union of elements from two or more words, made up of the beginning of the first and the end of the last, or, frequently, other combinations.”
Live and learn!