The bit is the basic unit of information. It represents a single choice between two equally likely possibilities, such as yes or no, true or false, or 0 and 1. Every piece of digital data, from a number to an image to a program, can ultimately be expressed as a sequence of bits.
The term was named in Claude Shannon’s 1948 paper “A Mathematical Theory of Communication.” Explaining how to measure information, Shannon wrote that the choice of a logarithmic base corresponds to the choice of a unit, and that “if the base 2 is used the resulting units may be called binary digits, or more briefly bits, a word suggested by J. W. Tukey.” Tukey was a colleague of Shannon’s, and the contraction of “binary digit” to “bit” stuck.
In Shannon’s framework a bit is not just a symbol on a wire but a measure of information. One bit is the amount of information gained from learning the answer to a single yes or no question whose two outcomes are equally likely. Messages with more uncertainty carry more bits; messages whose outcome is already known carry none.
This twin role, as both the smallest piece of digital data and the unit in which information is measured, makes the bit the atom of the digital world. Storage, communication speed, and the size of compressed files are all counted in bits and their multiples.