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
Students: Are you brave enough to let our tutors analyse your pronunciation?
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.
Students: We have free audio pronunciation exercises.
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 /

This is how I see the three words - Acronyms are necessarily abbreviations but many abbreviations are not acronyms. Thus, the common practice of saying "Abbreviations and acronyms" amounts to unnecessary repetition and should be avoided. It is enough to say abbreviations. According to the Oxford Advanced Learner's Dictionary, to abbreviate is to "make a word, phrase or name shorter by leaving out letters or using only the first letter of each word".

Concerning contractions and abbreviations, I find that they are markedly different. Abbreviations use first letters. They do not combine words and use, for instance, apostrophes to represent letters as in contractions.

FBI is an initialism
Site Hint: Check out our list of pronunciation videos.

An abbreviation is a shortened form of a written word or phrase. There are many different kinds of abbreviations: acronyms, initialisms, portmanteau, truncations and clipped words.

Acronyms are words formed from letters in a series of related words. If initials of a group of words are used to form a new word it is called an acronym.e.g. NATO.

Initialism is an abbreviation consisting of initial letters pronounced separately (e.g. BBC).

The difference between an acronym and initialism is that the abbreviation formed with initials is not pronounced as a word, rather you say the individual letters.