Apple Display Connector
From Mac Guides
ADC redirects here, and also stands for Apple Developer Connection
The Apple Display Connector (aka "ADC") is a connector for attaching monitors to Macintosh computers.
It integrates in a single connector:
ADC was introduced by Apple in 2000, and abandoned in 2004, when Apple decided to use DVI as the preferred display standard. (DVI-I on computers, DVI-D on displays.)
Computers with ADC ports can be connected to either VGA or DVI displays using inexpensive passive adapters. Macs that shipped without VGA-compatible video output included an ADC-to-VGA adapter.
Computers with DVI ports can be connected to those ADC-based displays that use the DVI-compatible signals. Adapters to do this tend to be expensive (over $150) because they must include a power supply.
ADC-based displays that use the VGA-compatible signals (some of Apple's CRT-based displays) can not be used with non-ADC computers, because an appropriate adapter was never manufactured.