In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system, where noise is defined as any signal other than the one being monitored.

In radio communication and electronics, this may include thermal noise, blackbody, cosmic noise as well as atmospheric noise from distant thunderstorms and similar and any other unwanted man-made signals, sometimes referred to as incidental noise. If the dominant noise is generated within the measuring equipment (for example by a receiver with a poor noise figure) then this is an example of an instrumentation noise floor, as opposed to a physical noise floor. These terms are not always clearly defined, and are sometimes confused.