Environmental psychology can mean one of two related things:
1) An interdisciplinary field focused on the interplay between humans and their surroundings. Various researchers have also termed this environmental social sciences, architectural psychology, socio-architecture, ecological psychology, ecopsychology, behavioral geography, environment-behavior studies, person-environment studies, environmental sociology, social ecology, and environmental design research.
2) The relationship between people's psychology, and their behavior in terms of sustainability.[verification needed] This is an unusual use of the term - choosing another term would be desirable for clarity.
This is discussed at the Appropedia page on Psychology.