Is it farther or further?
A reference that doesn't actually lead anywhere. In C and some other languages, a pointer that doesn't actually point at anything valid. Usually this happens because it formerly pointed to something that has moved or disappeared, e.g. a heap-allocated block which has been freed and reused.
Used as jargon in a generalisation of its technical meaning; for example, a local phone number for a person who has since moved is a dangling pointer.