The aim of this paper is to position, define and discuss three concepts crucial for research and practice concerning person-environment relationships, viz. accessibility, usability and universal design. Literature review, synthesized with the authors' research and practice experiences. The authors suggest an instrumental, three-step definition to accessibility, highlighting that accessibility comprises a personal as well as a environmental component, and that accessibility must be analysed by an integration of both. Suggesting the introduction of an activity component, accessibility should partly be replaced by the more complex term usability. Universal design is highlighted as a more process-oriented but less stigmatizing concept. This paper contributes to the positioning and definition of concepts describing person-environment relationships. The definitions suggested challenge current terminology, but can support in developing more efficient research and practice strategies. In order to develop theory for application to societal planning issues, the definition of concepts is a necessary step.