Refers to the conversion of electric power by using solid state components such as semiconductors. Can also generally apply to the field of electrical engineering which works with power electronics.
Power Electronics deals with the transformation of power to suit a needed purpose. Applications include switch-mode power supply design and motor drives.
Some assorted books on the subject:
Power Electronics: Converters, Applications, and Design Mohan et al. (Considered a standard undergrad power electronics textbook in the US.) Transformer and Inductor Design Handbook McLyman Switching Power Supply Design Pressman Switchmode Power Supply Handbook Billings Switching Power Supply Design and Optimization Maniktala