  We introduce a new tool for the quantitative characterisation of the
departure form Markovianity of a given dynamical process. Our tool can be
applied to a generic $N$-level system and extended straightforwardly to
Gaussian continuous-variable systems. It is linked to the change of the volume
of physical states that are dynamically accessible to a system and provides
qualitative expectations in agreement with some of the analogous tools proposed
so far. We illustrate its prediticve power by tackling a few canonical
examples.
