  The question of defining chaos is basically the question what makes a dynamical system such as (1) chaotic rather than nonchaotic. But this turns out to be a hard question to answer! Stephen Kellert defines chaos theory as “the qualitative study of unstable aperiodic behavior in deterministic nonlinear dynamical systems” (1993, p. 2). This definition restricts chaos to being a property of nonlinear dynamical systems (although in his (1993), Kellert is sometimes ambiguous as to whether chaos is only a behavior of mathematical models or of actual-world systems). That is, chaos is chiefly a property of particular types of mathematical models. Furthermore, Kellert’s definition picks out two key features that are simultaneously present: instability and aperiodicity. Unstable systems are those exhibiting SDIC. Aperiodic behavior means that the system variables never repeat any values in any regular fashion. I take it that the “theory” part of his definition has much to do with the “qualitative study” of such systems, so let’s leave that part for  §2.   Chaos, then, appears to be unstable aperiodic behavior in nonlinear dynamical systems. 