What term refers to a band of frequencies used for a particular purpose?

The term that refers to a band of frequencies used for a particular purpose is "channel." In telecommunications and electronic warfare, a channel is defined as a specific frequency or range of frequencies that is dedicated to transmitting signals for a certain application. This can include various forms of communication such as radio frequencies used for broadcasting, satellite communications, or any dedicated frequency range suitable for particular operational needs.

Channels are critical in managing how signals are sent and received, ensuring that multiple transmissions can occur without interference. This is especially important in environments like electronic warfare, where the effective use of channels can mean the difference between successful communication and signal interception.

The other terms provided have distinct meanings: "medium" generally refers to the physical substance through which signals are transmitted (like air or fiber optics), "bandwidth" describes the range of frequencies within a given band that can carry a signal (but does not alone specify the usage), and "signal" denotes the actual transmission of information, rather than the frequency range dedicated to its transmission. Understanding these definitions helps clarify why "channel" is the correct and relevant term for the context of the question.