This paper studies concept drift over time. We first define the meaning of a concept in terms of intension, extension and label. We then introduce concept drift over time and two derived notions: (in)stability over a time period and concept shift between two time points. We apply our framework in three case-studies, one from communication science, on DBPedia, and one in the legal domain. We describe ways of identifying interesting changes in the meaning of concept within given application contexts. These case-studies illustrate the feasibility of our framework in analysing concept drift in knowledge organisation schemas of varying expressiveness.