For the sake of argument an abbreviation is a shortened or contracted form of a word or phrase used the represent the whole, as Dr. for Doctor or U.S. For United States.
Initialisms and acronyms are two types of abbreviations that are used to shorten phrases.
Initialisms are abbreviations that are pronounced one letter at a time (FBI, BBC) while acronyms are abbreviations that are pronounced as words (NATO).
I should have stated that I was looking for the definition of those initialisms, but then no one would care...BTW the search engine didn't provide any clues.
Cheers,