Quiz: Remember the definition of mal de mer?
decimal part of a logarithm, 1865, from Latin mantisa "a worthless addition, makeweight," perhaps a Gaulish word introduced into Latin via Etruscan (cf. Old Irish meit, Welsh maint "size").
The part of a logarithm to the base ten that is to the right of the decimal point. For example, if 2.749 is a logarithm, .749 is the mantissa. Compare characteristic.