The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mantissa
    n 1: the positive fractional part of the representation of a
         logarithm; in the expression log 643 = 2.808 the mantissa
         is .808 [syn: mantissa, fixed-point part]

The Free On-line Dictionary of Computing (30 December 2018):

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)