Source : Webster's Revised Unabridged Dictionary (1913)

Null \Null\, a. [L. nullus not any, none; ne not + ullus any, a
   dim. of unus one; cf. F. nul. See {No}, and {One}, and cf.
   {None}.]
   Of no legal or binding force or validity; of no efficacy;
   invalid; void; nugatory; useless.

         Faultily faultless, icily regular, splendidly null,
         Dead perfection; no more.                --Tennyson.

Null \Null\, n.
   1. Something that has no force or meaning.

   2. That which has no value; a cipher; zero. --Bacon.

   {Null method} (Physics.), a zero method. See under {Zero}.

Null \Null\, v. t. [From null, a., or perh. abbrev. from annul.]
   To annul. [Obs.] --Milton.

Null \Null\, n. [Etymol. uncertain.]
   One of the beads in nulled work.

Source : WordNet®

null
     adj : lacking any legal or binding force; "null and void" [syn: {void}]

null
     n : a quantity of no importance; "it looked like nothing I had
         ever seen before"; "reduced to nil all the work we had
         done"; "we racked up a pathetic goose egg"; "it was all
         for naught"; "I didn't hear zilch about it" [syn: {nothing},
          {nil}, {nix}, {nada}, {aught}, {cipher}, {cypher}, {goose
         egg}, {naught}, {zero}, {zilch}, {zip}]

Source : Free On-Line Dictionary of Computing

null
     
         A special value used in several languages to
        represent the thing referred to by an uninitialised pointer.
     
         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.
     
        (2003-06-17)