Source : Webster's Revised Unabridged Dictionary (1913)

Assertion \As*ser"tion\, n. [L. assertio, fr. asserere.]
   1. The act of asserting, or that which is asserted; positive
      declaration or averment; affirmation; statement asserted;
      position advanced.

            There is a difference between assertion and
            demonstration.                        --Macaulay.

   2. Maintenance; vindication; as, the assertion of one's
      rights or prerogatives.

Source : WordNet®

assertion
     n 1: a declaration that is made emphatically (as if no supporting
          evidence were necessary) [syn: {averment}, {asseveration}]
     2: the act of affirming or asserting or stating something [syn:
         {affirmation}, {statement}]

Source : Free On-Line Dictionary of Computing

assertion
     
         1. An expression which, if false, indicates an
        {error}.  Assertions are used for {debugging} by catching
        {can't happen} errors.
     
        2. In {logic programming}, a new {fact} or {rule} added to the
        database by the program at {run time}.  This is an
        {extralogical} or impure feature of logic programming
        languages.
     
        (1997-06-30)