1. A shortened form, an abridgment, or a condensation of a longer publication: Every Reader's Digest condensed book is an abbreviation of an originally longer work.

Some abbreviation of Gary's speech was necessary because he didn't have enough time to deliver all of it.

2. Letters which are used to represent shortened forms or references to words or phrases: U.S.A., FBI, Calif., and Dr. are just a few abbreviations or shortened forms of words or phrases to represent the full terms.
3. Letters that are shortened references for a word or a phrase and which themselves are pronounceable as words: NATO, NASA, and UNICEF are types of abbreviations called acronyms; or abbreviations that may be pronounced as a word rather than just as a series of letters.

Remember: an abbreviation is pronounced as separate letters (U.S.A., FBI) while an acronym may be pronounced as a word (NATO, NASA).

4. Etymology: from Middle French abréviation, from Late Latin abbreviationem, abbreviatio, from abbreviare "to make brief" from Latin ad, "to" + breviare "to shorten" from brevis "short, low, little, shallow".