True-believer syndrome is a common occurrence in parapsychology. It is, broadly, a belief without logical foundations. Frequently, this translates to someone believing so strongly in the truth of something, that they refuse to accept when it is proven wrong, even if this proof is irrefutable. Just as skeptics are often too quick to dismiss, supporters often take the other route, and refuse to dismiss in light of evidence.
This occurs frequently in paranormal phenomenon with the staging of events. Someone may become so convinced that an encounter with a UFO, apparition, or cryptozoological figure is real, that they tend not to believe it to be a hoax, even if the original author of the evidence makes the statement.
This concept also occurs completely outside of parapsychology. For example, when a fraud is committed, many victims continue to profess the good faith of the scammer. The term is, however, rarely used outside of parapsychology.
The term was coined by M. Lamar Keene in 1976, in Keene's The Psychic Mafia. Keene attributed the success of many mediums to this. Ironically, people apparently suffering from true-believer syndrome were Keene's downfall. He had originally been a psychic, and in The Psychic Mafia, stated that his practice was fake. The book caused such a firestorm that Keene left the psychic business. However, death threats continued to follow, culminating in a drive-by shooting. Keene subsequently changed his name and relocated, and his whereabouts are unknown.