ABC can stand for many things. Here are some of the most common full forms:

• Activity-Based Costing - An accounting method that assigns costs to activities and then to products or services based on their consumption of those activities. Source
• American Broadcasting Company - A major commercial broadcast television network in the United States. Source
• Audit Bureau of Circulations - An organization that verifies and reports circulation data for publications.
• Australian Broadcasting Corporation - The principal public broadcaster in Australia. Source
• Atanasoff-Berry Computer - Considered by some to be the first electronic digital computer. Source

The specific meaning of ABC depends on the context in which it is used.