Megamodel is a model of MDE,  which explains what is a model, a metamodel, a transformation, but also what is a transformation model, a model transformation, a model of model, a meta-model of transformation, and any combination of these terms. The figure from link 1.

MegModel_ELUsing megamodel it is possible to formulate model transactions, simulating the model,  and making queries, abstractions and specializatios based on model information.

Some links:

  1. Jean-Marie Favre and Tam NGuyen. Towards a Megamodel to Model: Software Evolution Through Transformations.
  2. Atlas, Megamodel Management.