What are the differences between abbreviation, contraction and acronym?

I have looked up in a dictionary and here is what it says:

Contraction: A word, as won't from will not, or phrase, as o'clock from of the clock, formed by omitting or combining some of the sounds of a longer phrase.

Abbreviation: A shortened form of a word or phrase used chiefly in writing to represent the complete form, such as Mass. for Massachusetts or USMC for United States Marine Corps.

Acronym: A word formed from the initial letters of a name, such as WAC for Women's Army Corps, or by combining initial letters or parts of a series of words, such as radar for radio detecting and ranging.

Isn't abbreviation a subset of contraction based on the definitions provided? Also, acronym is quite similar to abbreviation, for example, USMC and WAC? Could someone please clarify the differences? Thanks
acronym: just initials
abbreviation: shortening (cut some of the final letters, etc)
contraction: some of the middle letters can be removed too

Simply put, an acronym is an abbreviation that is pronounced as a word.

eg FBI is an abbreviation. We pronounce each letter separately, thus eff bee eye

eg NATO is an acronym. We pronounce it as a word naytoe, not as enn eh tee oh

So, USMC is an abbreviation but not an acronym.

best wishes, Clive
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So, USMC for United States Marine Corps is both a abbreviation and also acronym? Can I say acronym is a special case of abbreviation since acronym is abbreviated and left with only initials?
 Clive's reply was promoted to an answer.
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New2grammarWhat are the differences between abbreviation, contraction and acronym?
An acronym is a set of letters that can be pronounced as a single word (e.g., NATO pronounced "NAY-TOE") whereas an abbreviation is pronounced one letter at a time (e.g., CSS pronounced "SEE ESS ESS").

source : http://mondaybynoon.com/2007/01/08/abbreviation-or-acronym-whats-the-difference /