The logarithm of the reciprocal of a number, equal to the negative of the common logarithm; used in mathematical and astronomical calculations to simplify division problems.
From 'co-' (complement, reciprocal) + 'logarithm' (from Greek 'logos' meaning ratio + 'arithmos' meaning number). This term emerged in 17th-century mathematics to streamline computational methods.
Cologarithms were a brilliant hack: instead of dividing, you could look up a colog in a table and subtract—it turned a hard operation into an easy one, which is why mathematicians and surveyors loved them until electronic calculators made the whole system obsolete.
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