Jump to navigation Jump zero binary copy option search This article is about the unit of information. The bit is a basic unit of information in information theory, computing, and digital communications. In information theory, one bit is typically defined as the information entropy of a binary random variable that is 0 or 1 with equal probability, or the information that is gained when the value of such a variable becomes known.
As a binary digit, the bit represents a logical value, having only one of two values. It may be physically implemented with a two-state device. The symbol for the binary digit is either simply bit per recommendation by the IEC 80000-13:2008 standard, or the lowercase character b, as recommended by the IEEE 1541-2002 and IEEE Std 260. Ralph Hartley suggested the use of a logarithmic measure of information in 1928. A bit can be stored by a digital device or other physical system that exists in either of two possible distinct states.
Bits can be implemented in several forms. In most modern computing devices, a bit is usually represented by an electrical voltage or current pulse, or by the electrical state of a flip-flop circuit. The specific voltages are different for different logic families and variations are permitted to allow for component aging and noise immunity. Bits are transmitted one at a time in serial transmission, and by a multiple number of bits in parallel transmission. A bitwise operation optionally processes bits one at a time. In the earliest non-electronic information processing devices, such as Jacquard’s loom or Babbage’s Analytical Engine, a bit was often stored as the position of a mechanical lever or gear, or the presence or absence of a hole at a specific point of a paper card or tape. In modern semiconductor memory, such as dynamic random-access memory, the two values of a bit may be represented by two levels of electric charge stored in a capacitor.