Definition - A word formed from the first letters of other words. What make it an initialism is that it is pronounced as a sequence of letters.
Note: As opposed to acronyms, which are also words formed from the first letters of other words, but which when spoken are pronounced as words.

Example -

(1) HIV
(2) BBC

Etymology -
The word was coined by combining the word initial with the suffix ism. The former derives from the Latin initium, a beginning or an entrance.

Oxford English Dictionary -
The term's first citation is from 1899:
"In my ‘Handbook’ I give an initialism of Mr. Watts's, ‘P. P. C. R.’"
(R. Thomas in N. & Q. 9th Ser. III. 103/1)

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