The self-information, also known as the information content of a signal, random variable, or event is defined as the negative logarithm of the probability of the given outcome occurring.
When applied to a discrete random variable, the self-information can be represented as
is the relative entropy of the probability distribution  from a Kronecker delta representing certainty that  — i.e. the number of extra bits that must be transmitted to identify  if only the probability distribution  is available to the receiver, not the fact that .