- H+ concentration
- hydrogen-ion concentration
- A measure of acidity and alkalinity of a solution that is a number on a scale on which a value of 7 represents neutrality and lower numbers indicate increasing acidity and higher numbers increasing alkalinity and on which each unit of change represents a tenfold change in acidity or alkalinity and that is the negative logarithm of the effective hydrogen-ion concentration or hydrogen-ion activity in gram equivalents per liter of the solution (instead of saying that the concentration of hydrogen ion in pure water is 1.00 x 10-7, it is customary to say that the pH of pure water is 7--Linus Pauling); also: the condition represented by such a number.
Definition from: Merriam-Webster's Medical Dictionary by Merriam-Webster Inc.
Related discussion in the Handbook
See also Understanding Medical Terminology.