  We demonstrate that the metamaterial perfect absorber behaves as a
meta-cavity bounded between a resonant metasurface and a metallic thin-film
reflector. The perfect absorption is achieved by the Fabry-Perot cavity
resonance via multiple reflections between the "quasi-open" boundary of
resonator and the "closed" boundary of reflector. The characteristic features
including angle independence, ultra-thin thickness and strong field
localization can be well explained by this model. With this model, metamaterial
perfect absorber can be redefined as a meta-cavity exhibiting high Q-factor,
strong field enhancement and extremely high photonic density of states, thereby
promising novel applications for high performance sensor, infrared
photodetector and cavity quantum electrodynamics devices.
