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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.