A
bit or
binary digit is the basic unit of
information in
computing and
telecommunications; it is the amount of information that can be stored by a digital device or other physical system that can usually exist in only two distinct
states. These may be the two stable positions of an
electrical switch, two distinct
voltage or
current levels allowed by a
circuit, two distinct levels of
light intensity, two directions of
magnetization or
polarization, etc.
In
computing, a bit can also be defined as a
variable or computed quantity that can have only two possible
values. These two values are often interpreted as
binary digits and are usually denoted by the Arabic
numerical digits 0 and 1. Indeed, the term "bit" is a contraction of
binary digit. The two values can also be interpreted as
logical values (
true/
false,
yes/
no), algebraic
signs (
+/
−), activation states (
on/
off), or any other two-valued attribute. In several popular programming languages, numeric 0 is equivalent (or convertible) to logical
false, and 1 to
true. The correspondence between these values and the physical states of the underlying
storage or
device is a matter of convention, and different assignments may be used even within the same device or
program.
In
information theory, one bit is typically defined as the uncertainty of a binary random variable that is 0 or 1 with equal probability,
[1] or the information that is gained when the value of such a variable becomes known.
[2]
In
quantum computing, a
quantum bit or
qubit is a
quantum system that can exist in
superposition of two bit values, "true" and "false".
The symbol for bit, as a unit of information, is either simply "bit" (recommended by the
ISO/IEC standard 80000-13 (2008)) or lowercase "b" (recommended by the
IEEE 1541 Standard (2002)).
Multiple bits
There are several
units of information which are defined as multiples of bits, such as
byte (8 bits),
kilobit (either 1000 or 2
10 = 1024 bits),
megabyte (either
8000000 or 8×2
20 =
8388608bits), etc.
Computers usually manipulate bits in groups of a fixed size, conventionally named "
words". The number of bits in a word varies with the computer model; typically between 8 to 80 bits; or even more in some specialized machines.
The
International Electrotechnical Commission's standard
IEC 60027 specifies that the symbol for binary digit should be "bit", and this should be used in all multiples, such as "kbit" (for kilobit).
[5] However, the letter "b" (in lower case) is widely used too. The letter "B" (upper case) is both the standard and customary symbol for byte.
In telecommunications (including
computer networks), data transfer rates are usually measured in
bits per second (bit/s) or its multiples, such as kbit/s. (This unit is not to be confused with
baud.)
