In the context of radiation emission from an electron beam, Dicke's superradiance (SR) is the enhanced "coherent" spontaneous radiation emission from a pre-bunched beam, and Stimulated-Superradiance (ST-SR) is the further enhanced emission of the bunched beam in the presence of a phase-matched radiation wave. These processes are analyzed for Undulator radiation in the framework of radiation field mode-excitation theory. In the nonlinear saturation regime the synchronism of the bunched beam and an injected radiation wave may be sustained by wiggler tapering: Tapering-Enhanced Superradiance (TES) and Tapering-Enhanced Stimulated Superradiance Amplification (TESSA). Identifying these processes is useful for understanding the enhancement of radiative emission in the tapered wiggler section of seeded FELs. The nonlinear formulation of the energy transfer dynamics between the radiation wave and the bunched beam fully conserves energy. This includes conservation of energy without radiation reaction terms in the interesting case of spontaneous self-interaction (no input radiation).