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A binary unit of information, numerically equal to the amount of information obtained during a trial with two mutually exclusive equally probable alternatives ( $  p _ {1} = p _ {2} = 1/2 $):

$$ 
I (p _ {1} , p _ {2} )  = \ 
p _ {1}   \mathop{\rm log} _ {2}  p _ {1} -
p _ {2}   \mathop{\rm log} _ {2}  p _ {2}  = \ 
 \mathop{\rm log} _ {2}  2  =  1 \ \ 
\textrm{ \BQT bit\EQT  } ,
$$

the logarithms being taken to the base 2. The bit is the most frequently used unit, but other units of information are also employed — the  "Hartley"  or the  "nit" , the definitions of which involve decimal and natural logarithms respectively.

====Comments====
The definition as given comes from information theory. In computer science the term  "bit"  usually refers to the representation of  "0"  or  "1"  by a suitable physical device which can be (exclusively) in two alternative states, or even to that device itself. Nits or Hartley's are unknown in the West.