ABC

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The alphabet. Often used in the plural: learned her ABCs when she was three years old.
2. n. The rudiments of reading and writing. Often used in the plural.
3. n. The rudiments of a subject. Often used in the plural: taught the ABCs of geometry.

Century Dictionary and Cyclopedia

1. n. The first three letters of the alphabet; hence, the alphabet.
2. n. An a-b-c book; a primer.

Wiktionary

1. n. UK, rail transport A British alphabetized guidebook for trains and their stations.
2. ABC (B), a Belgian car market.
3. activity-based costing.
4. African Basketball Confederation.
5. Airway, breathing, and circulation.
6. Alcoholic beverage control.
7. already been chewed, as in chewing gum.
8. American Book Center.
9. American-born Chinese.
10. American Broadcasting Company.
11. medicine Antigen binding capacity.
12. anything but Chardonnay: a backlash against Chardonnay wine, seen as ubiquitous.
13. Japan Asahi Broadcasting Corporation.
14. Atlanta, Birmingham & Coast Railroad.
15. atomic, biological, and chemical.
16. Aural brevity code.
17. Australian-born Chinese.
18. 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.

GNU Webster's 1913

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

WordNet 3.0

1. n. the elementary stages of any subject (usually plural)