Binary digit: The smallest unit of data in a digital system, with a value of either 0 or 1. A group of bits, such as 8-bits or 16-bits, compose a byte. The number of bits in a byte depends upon the processing system being used. Whenever you see a lowercase b associated with a number, it's likely to be a bit. It can be prefixed with kilo- (for 1,024 bits, or 2 to the 10th power) or mega- (1,024 x 1,024 bits)--and sometimes finds its way into data transfer speeds (such as 14.4 kbps).