  Variable length communication over a compound channel with feedback is
considered. Traditionally, capacity of a compound channel without feedback is
defined as the maximum rate that is determined before the start of
communication such that communication is reliable. This traditional definition
is pessimistic. In the presence of feedback, an opportunistic definition is
given. Capacity is defined as the maximum rate that is determined at the end of
communication such that communication is reliable. Thus, the transmission rate
can adapt to the channel chosen by nature. Under this definition, feedback
communication over a compound channel is conceptually similar to multi-terminal
communication. Transmission rate is a vector rather than a scalar; channel
capacity is a region rather than a scalar; error exponent is a region rather
than a scalar. In this paper, variable length communication over a compound
channel with feedback is formulated, its opportunistic capacity region is
characterized, and lower bounds for its error exponent region are provided..
