Usually, the binary number system, i.e. the positional number system with base 2. This is the number system most commonly used in computers. A binary digit (or bit) is either 0 or 1. The representation of numbers by binary digits is called binary notation.
The term binary system is also used to describe any system in which there are just two possible states. For example, each of the elements comprising the memory of any computer is a binary system, one of whose states is used to denote the binary digit 0 and the other to denote the binary digit 1. It is customary to refer to such a storage element, or to the unit of information in any binary system, as a bit.