When an apostrophe replaces missing letters, the result is called a contraction.
Rock 'n Roll or more properly Rock 'n' Roll but that's not the common practice.
Int'l (for international)
When the word is shortened, by dropping letters from the end, it's called an abbreviation.
Corp. (for corporation).
Eng. (for English or Engineer)
Then there is the in-between area that no one can decide on. When letters are dropped from the middle, and no apostrophe appears:
Or what about entirely irregular things like
Mrs. (where did that R come from!?)
The British have a really nice system that I wish were American/Cdn standard: If the word is simply shortened, they use a period/full stop. If it's contracted, they don't. I like this:
Mr and Mrs Smith and Prof. Jones were itroduced to each other at Gov. Gen. Appleby's residence by The Hon. Appleby himself.
I like that because of the simple distinction between Street (St) and Saint (St.).
The other type is called an Initialism, if it is pronounced as each letter:
or an Acronym is you say it like a word:
until eventually the acronyms become words in their own right