The paper examines the semantics of the terms ‘environment’ and ‘habitat’ and presents a semi-formalised ontological framework in which these concepts are related to the spatial and material structure of the world. Since habitats are essentially associated with lives and behaviour of animals and plants the framework incorporates an abstract model of an organism. This enables the spatial extensions of habitats to be characterised in terms of the possible ‘life trajectories’ of organisms constrained by the physical properties of a geographic region and by the biological requirements for their survival.