ABC

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The alphabet.
• noun The rudiments of reading and writing.
• noun The rudiments of a subject.

from The Century Dictionary.

• noun The first three letters of the alphabet; hence, the alphabet.
• noun An a-b-c book; a primer.

from the GNU version of the Collaborative International Dictionary of English.

• The first three letters of the alphabet, used for the whole alphabet.
• obsolete A primer for teaching the alphabet and first elements of reading.
• The simplest rudiments of any subject.
• a primer.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun UK, rail transport A British alphabetized guidebook for trains and their stations.
• initialism ABC (B), a Belgian car market.
• initialism activity-based costing.
• initialism African Basketball Confederation.
• initialism Airway, breathing, and circulation.
• initialism Alcoholic beverage control.
• initialism already been chewed, as in chewing gum.
• initialism American Book Center.
• initialism American-born Chinese.
• initialism American Broadcasting Company.
• initialism medicine Antigen binding capacity.
• initialism anything but Chardonnay: a backlash against Chardonnay wine, seen as ubiquitous.
• initialism Japan Asahi Broadcasting Corporation.
• initialism Atlanta, Birmingham & Coast Railroad.
• initialism atomic, biological, and chemical.
• initialism Aural brevity code.
• initialism Australian-born Chinese.
• initialism An acrostic for Santo André, São Bernardo do Campo and São Caetano do Sul, satellite cities around the city of São Paulo that form the most important industrial area in the country.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the elementary stages of any subject (usually plural)