This chapter presents some basic radar definitions and establishes much of the nomenclature used throughout this text. The word radar is an abbreviation for radio detection and ranging. In most cases, radar systems use modulated waveforms and directive antennas to transmit electromagnetic energy into a specific volume in space to search for targets. Objects (targets) within a search volume will reflect portions of the incident energy (radar returns or echoes) in the direction of the radar. These echoes are then processed by the radar receiver to extract target information such as range, velocity, angular position, and other target identifying characteristics.