What term describes the ability of a system to operate on more than one frequency without simultaneous use?

The term that describes the ability of a system to operate on more than one frequency without simultaneous use is known as "Frequency Agile." This concept refers to a system's capability to switch between different frequency channels or bands while transmitting or receiving signals. Such flexibility is crucial in various applications, especially in military communications or radar systems, where the ability to adapt to different frequencies can enhance performance, reduce interference, and improve the likelihood of successful communication or detection.

In contrast, other terms do not convey this capability. For example, "Doppler" typically pertains to the frequency shift of a wave in relation to an observer moving relative to the source of the wave, which is distinct from the idea of operating on multiple frequencies independently. "Frequency Decoy" relates to tactics used to confuse or mislead guidance systems, while "Chaff" refers to material used to obscure radar signatures, not to the functionality of operating across multiple frequencies. These definitions highlight why "Frequency Agile" is the correct choice, as it directly encapsulates the capability being described in the question.