I am reading Pan et al. (2000), a paper about dihydrofolate reductase (DHFR). They claim using a ensemble-based model of DHFR.
What is a ensemble-based model?
An 'ensemble' is a set of related models of a structure.
When NMR is used to determine a structure, the result is an ensemble of structures (see this paper for a comparison with crystallography). However, it is possible to see multiple structures for crystallography as well. This paper on "Accessing protein conformational ensembles using room-temperature X-ray crystallography" describes limitations to the idea of crystal structures having one unique structure.