A device that is capable of directly displaying colors in a specific format without having to reduce the number of colors by software is referred to as a deep color device.
For instance 15 bit (32,768 colors), 16 bit (65,536 colors) and 24 bit (16.7 million colors) graphics adapters are deep color devices. These adapters can directly display 24-bit RGB data, so programs like PMView do not need to do any color reduction.
24-bit deep color is usually called true color. However, some people use true color interchangeably with deep color.