Abbreviations, Acronyms, and Initialisms

A - I                J - R                 S - Z

An ABBREVIATION is a shortened form of a word or phrase. 

An ACRONYM is a type of abbreviation formed from the initial letter or letters of a phrase or compound-word and pronounced as a single word.  Some examples are AWOL, laser, NATO, AIDS, hazmat, UNICEF, and NASA.

An INITIALISM is also a type of abbreviation formed from initial letters, but one that is not pronounced as a word.  Some examples are FBI, CIA, USA, FM, and YMCA.

Abbreviations by category:

Titles of Literary Works
Books of the Bible
Legal Terms and Abbreviations
Mathematical Terms and Symbols
Musical Terms
Periodic Table of Elements
Weights and Measures