2600

1. The frequency in hertz that AT&T originally used to indicate when a line is free. Some individuals took advantage of knowing this information by using whistles and other devices to generate 2600 hertz to make long distance calls. These individuals came to be known as phreaks.
2. 2600 is also a commonly used name for news or information containing information about hacking and phreaking. 

Also see: Phreak, Security definitions