Wiktionary
n. ionizing radiation that is naturally present in the environment.
WordNet
n. radiation coming from sources other than those being observed
Wikipedia
Background radiation is the ubiquitous ionizing radiation present in the environment. Background radiation originates from a variety of sources, both natural and artificial. Sources include cosmic radiation, naturally occurring radioactive materials such as radon, and fallout from nuclear weapons testing and nuclear accidents.
Usage examples of "background radiation".
That gives us a possible alternative explanation for the three-degree thermal background radiation, so maybe we don't need the Big Bang model to account for it at all now.
We've already seen that the wide-spread acceptance of the background radiation was a peculiar business, since it had been predicted more accurately without any Big Bang assumptions at all.
The background radiation levels showed an elevated background neutron count, as if from induced radioactivity.
However, I and several other people showed that, at least in its original form, it predicted much greater variations in the temperature of the microwave background radiation than are observed.
Though it might seem foolish to admit such dangerous wavelengths as the gamma and X ray wavelengths, the common decision has heen that it is far better to expose oneself to a little background radiation than it is to have one's shield emitting dangerous radiation during a fight, possibly endangering unshielded bystanders.
The remnants of that fireball, the cosmic background radiation, emanating from all parts of the sky can be detected by radio telescopes today.
I've got some news for you: the cosmic background radiation is getting hotter.
But not much, compared to what we already added to the background radiation.
It imposes a slight Doppler shift on the microwave background radiation.