Complex sentence: The term cutoff frequency or corner frequency is applicable to a wide range of systems, and represents a boundary in the system response at which energy entering the system begins to be attenuated or reflected instead of transmitted.
Split into simpler sentences: The term cutoff frequency is applicable to a wide range of systems. The cutoff frequency of a system is a boundary in the input spectrum at which energy entering the system begins to be attenuated or reflected instead of transmitted.