Scientists from many disciplines increasingly use the term ‘socio-ecological system’ to describe coupled human-environment systems. We offer the following specific definition to help practitioners better conceptualize what a socio-ecological system is: 1) a coherent system of biophysical and social factors that regularly interact, 2) a system that is defined at several spatial, temporal, and organizational scales which may be hierarchically linked, 3) a set of critical resources (natural, socioeconomic, and cultural) whose flow and use is regulated by a combination of ecological and social systems, and 4) a perpetually dynamic system with continuous adaptation.