Cologarithm

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.

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

In chemistry, a decimal cologarithm is indicated by the letter p (originally the Greek letter ρ).
For example, pK = – log₁₀ K and pH = – log₁₀ [H₃O⁺].

References

Meriam Webster definition of cologarithm
"Cologarithm." From MathWorld--A Wolfram Web Resource.