1. Software which, when installed by a user insufficiently enlightened to avoid it, enables third parties to snoop the user's hard drive or monitor their network transactions. Though the term seems to have entered use in the late 1990s, it achieved real popularity as applied to Microsoft Windows XP. Some back door features in XP permit Microsoft to (for example) covertly scan your disk directories for the names of files it might deem to be warez.
2. Systems for spying on email and web traffic, such as the FBI's Carnivore.