The belief or mentality that agents of communism are hiding amongst the greater populace, waiting for an opportunity to spread their agenda. An allusion to the paranoia that gripped America and other countries during the Cold War. Sometimes hyphenated if used before a noun. Despite the collapse of the Soviet Union so many years ago, there is still a mentality in the country that there are reds under the bed, so espousing a belief in anything resembling socialism is likely to make you a political pariah. The senator's reds-under-the-bed rhetoric is woefully outdated today.