  Shannon defined channel capacity as the highest rate at which there exists a
sequence of codes of block length $n$ such that the error probability goes to
zero as $n$ goes to infinity. In this definition, it is implicit that the block
length, which can be viewed as the number of available channel uses, is
unlimited. This is not the case when the transmission power must be
concentrated on a single transmission, most notably in military scenarios with
adversarial conditions or delay-tolerant networks with random short encounters.
A natural question arises: how much information can we transmit in a single use
of the channel? We give a precise characterization of the one-shot capacity of
discrete channels, defined as the maximum number of bits that can be
transmitted in a single use of a channel with an error probability that does
not exceed a prescribed value. This capacity definition is shown to be useful
and significantly different from the zero-error problem statement.
