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A bit refers to a digit in the binary numeral system (base 2). For example, the number 1001011 is 7 bits long. The unit is sometimes abbreviated to b (but see below).
Binary digits are almost always used as the basic unit of information storage and communication in digital computing and digital information theory. Information theory also often uses the natural digit, called either a nit or a nat.
History and explanation
Claude E. Shannon first used the word bit in a 1948 paper. Shannon's bit is a portmanteau word for binary digit (or possibly binary unit). He attributed its origin to John W. Tukey.
A bit is like a light switch; it can be either on or off. A single bit is a one or a zero, a true or a false, a "flag" which is "on" or "off", or in general, the quantity of information required to distinguish two mutually exclusive states from each other.
The bit is the smallest unit of storage currently used in computing, although much research is ongoing in quantum computing with qubits.
More than one bit
A byte is a collection of bits, originally variable in size but now almost always eight bits. Eight-bit bytes, also known as octets, can represent 256 values (28 values, 0–255). A four-bit quantity is known as a nibble, and can represent 16 values (24 values, 0–15).
"Word" is a term for a slightly larger group of bits, but it has no standard size. In the IA-32 architecture, 16 bits are called a "word" (with 32 bits being a "double word" or dword), but other architectures have word sizes of 32, 64 or others.
Terms for large quantities of bits can be formed using the standard range of prefixes, e.g., kilobit (kbit), megabit (Mbit) and gigabit (Gbit). Note that much confusion exists regarding these units and their abbreviations, see binary prefixes. It has often been recommended to use "bit" for the bit and "b" for the byte, to prevent confusion with the unit bel, B. However, "b" is often used for bit and "B" for byte. The IEC recommends to use only "bit" and "B" for maximum disambiguation. Since the bel is almost never used by itself (only used as a decibel, dB) the chances of conflict are small.
Certain bitwise computer processor instructions (such as xor) operate at the level of manipulating bits rather than manipulating data interpreted as an aggregate of bits.
Telecommunications or computer network transfer rates are usually described in terms of bits per second (not to be confused with baud).