What is the difference between these initialisms vs acronyms vs abbreviations?
As I understand from SO:
initialisms are like BOA,USA(i.e. letter by letter). acronyms are like NASA, AIDS(i.e. words).
than what is abbreviations?
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An abbreviation shortens one word or a short sequence of words to one or a few letters. A few random examples:
As you can see, in the wider definition abbreviations encompasses contractions ("dropping letters from the middle of a word", acronyms and initialisms (using the first letter(s) of a multi-word phrase) and abbreviations in the stricter sense, which are defined as "dropping letters from the end of a word".