A branch of mathematics dealing with nonlinear systems, which are drastically affected even by very small changes in their initial values (e.g., the flap of a butterfly's wings may be enough to affect the path of a storm system some weeks later). Like the weather, behaviour is a nonlinear system. For measurement at any level of precision, the mathematics of chaos demonstrates that, although we can predict kinds of things that will happen, we cannot predict specific details (e.g., we can predict that a pigeon will peck a key, but not precisely when). This makes interpretation far more important, and in many behavioural applications it will be all that is feasible.