mantissae

Mantissae is the plural of mantissa, a term used in mathematics and computing with several related meanings. The word comes from Latin mantissa, meaning "something to be counted" and entered English via early mathematical literature.

In the context of logarithms, the mantissa is the fractional part of a logarithm. If log_b(x) = k + f, with k an integer (the characteristic) and f the fractional part, then f is called the mantissa. The mantissa usually lies in the interval [0, 1) for common bases such as base 10. Some older sources use mantissa more broadly to refer to the entire logarithm, but modern terminology typically distinguishes the integral part (characteristic) from the fractional part (mantissa).

In floating-point representation, the term mantissa is used for the part of a number that contains its significant digits, together with an exponent: x ≈ m × b^e, where m is the mantissa or significand. In contemporary practice, the preferred term is significand for the digits, and mantissa is considered outdated or ambiguous; however, many programming languages and legacy texts still use mantissa.

The plural form mantissae is found in some writings, reflecting its Latin origin; mantissas is also used as an English plural. The term is specialized and most common in historical discussions of logarithms and in discussions of older computing literature.