<programming> A special value used in several languages to represent the thing referred to by an uninitialised pointer.
<database> A special value that may be stored in some database columns to represent an unknown, missing, not applicable, or undefined value. Nulls are treated completely differently from ordinary values when evaluating SQL expressions and there are several SQL constructs for dealing with nulls.
Try this search on Wikipedia, OneLook, Google
Nearby terms: NUCLEOL « nude « nuke « null » nullary » null modem » null-terminated multibyte string