1929 Flying boat family by Hall.
(From potential of Hydrogen) the logarithm of the reciprocal of hydrogen-ion concentration in gram atoms per liter; provides a measure on a scale from 0 to 14 of the acidity or alkalinity of a solution (where 7 is neutral and greater than 7 is more basic and less than 7 is more acidic);
See moreAlgorithmic complexity class; the union of all complexity classes in the polynomial hierarchy; the set of languages expressible by second-order logic.