What is "cologarithm"

n. (context mathematics English) The logarithm of the reciprocal of a number (equal to the negative of the original logarithm)

In mathematics, the base-b cologarithm, sometimes shortened to colog, of a number is the base-b logarithm of the reciprocal of the number. It is equal to the negative base-b logarithm of the number.

$$\operatorname{colog}_b\ x = \log_b \left(\frac{1}{x} \right) = \log_b 1-\log_b x = -\log_b x \,$$

In chemistry, a decimal cologarithm is indicated by the letter p. This usage originated with the quantity pH, defined as –log [HO]. Based on pH, the quantity pK was later defined as –logK.