Source : Webster's Revised Unabridged Dictionary (1913)

Mantissa \Man*tis*sa\, n. [L., an addition, makeweight; of
   Tuscan origin.] (Math.)
   The decimal part of a logarithm, as distinguished from the
   integral part, or characteristic.

Source : WordNet®

mantissa
     n : the positive fractional part of the representation of a
         logarithm; in the expression log 643 = 2.808 the mantissa
         is .808 [syn: {fixed-point part}]

Source : Free On-Line Dictionary of Computing

mantissa
     
        1.  The part of a {floating point} number which,
        when multiplied by its {radix} raised to the power of its
        {exponent}, gives its value.  The mantissa may include the
        number's sign or this may be considered to be a separate part.
     
        2.  The fractional part of a {logarithm}.
     
        (1996-06-15)