Using deuteron NMR techniques two-, effective three-, and various four-time correlation functions were recorded for supercooled ortho-terphenyl at 10-15 K above the calorimetric glass transition in order to characterize the heterogeneous nature of its primary response. The experimental results could successfully be described within various energy landscape models as well as via continuous time random walk simulations. These theoretical considerations provide a suitable basis far a definition of the term dynamic heterogeneity. We discuss the power but also some limitations of the present multidimensional NMR techniques when applied to amorphous materials.