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.

In radio communication and electronics, this may includes thermal noise, blackbody, and any other interfering signals. In a measurement system such as a seismograph, it may include nearby foot traffic or a nearby road. The noise floor limits the smallest measurement that can be taken since any measured amplitude can be no less than the noise floor.

A common way to lower the noise floor in electronics systems is to cool the system to reduce thermal noise, which is usually the major noise source.