shannon

(Sh)

   a unit of information content used in information and communications theory.

   The definition is based on the idea that less-likely messages are more informative than more-likely ones (for example, if a volcano rarely erupts, then a message that it is erupting is more informative than a message it is not erupting). If a message has probability p of being received, then its information content is -log² p shannons. For example, if the message consists of 10 letters, and all strings of 10 letters are equally likely, then the probability of a particular message is 1/26¹⁰ and the information content of the message is 10(log² 26) = 47.004 shannons. This unit was originally called the bit², because when the message is a bit string and all strings are equally likely, then the information content turns out to equal the number of bits. One shannon equals log¹⁰ 2 = 0.301 030 hartley or log^e 2 = 0.693 147 nat. The unit is named for the American mathematician Claude Shannon (1916-2001), the founder of information theory.