1 Very simply, a continuum representing two variables: A and B. The continuum ranges from 100% A, 0% B, to 0% A, 100% B. See, for example, Hooke (2003) TIBG 28, 2 on the phase space of meander movement and bend curvature. Melton (1958) J. Geol. 66 describes a mature drainage basin phase space with four variables.
2 Very much less simply, an abstract concept that captures all the possible spaces in which a spatio-temporal system might exist in theoretical terms; not just what happens but what might happen; ‘the space of the possible’ (J. Cohen and I. Stewart 1994). ‘The geography of phase space is flexible, but not totally arbitrary: the main possibilities are “already there”, constrained by contextual realities’ (I. Stewart and J. Cohen 1997). This use of the concept is not totally distinct from 1; there are just many more factors involved. Martin (in Harrison et al. (2006) Area 38, 4) draws on Richards in J. Matthews and D. Herbert (2004) to see phase space as ‘a middle road between relational space and more fixed notions of spatiality’.
Hooke (2007) Geomorph. 91, 3–4 uses phase space plots to help uncover emergent behaviour in meandering rivers.