8 letters · 10 points in Scrabble
noun the positive fractional part of the representation of a logarithm; in the expression log 643 = 2.808 the mantissa is .808
originBorrowed from Latin mantissa, mantīsa, of unknown origin (said by Festus to be a loanword from Etruscan, but it may actually be from Celtic, possibly through Etruscan mediation).
Etymology from Wiktionary, CC BY-SA 3.0