  In this paper we propose, discuss and illustrate a computationally feasible
definition of chaos which can be applied very generally to situations that are
commonly encountered, including attractors, repellers and non-periodically
forced systems. This definition is based on an entropy-like quantity, which we
call "expansion entropy", and we define chaos as occurring when this quantity
is positive. We relate and compare expansion entropy to the well-known concept
of topological entropy, to which it is equivalent under appropriate conditions.
We also present example illustrations, discuss computational implementations,
and point out issues arising from attempts at giving definitions of chaos that
are not entropy-based.
