So the word mantissa has two different meanings that show up in really similar contexts. In

  log(120) = log(1.2 × 10²) = 2.079,

you could call both the .079 and the 1.2 a mantissa. The first is in the range [0,1) and the second, aka significand, is in [1,10) for base 10.

And really one mantissa is just the log of the other.