Webster's English Dictionary

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ph

pH \pe--'a-ch\ n : the negative logarithm of the effective hydrogen-ion 
   concentration or hy drogen-ion activity in gram equivalents per liter used 
   in expressing both acidity and alkalinity on a scale whose values run from 
   0 to 14 with 7 representing neutrality, numbers less than 7 increasing 
   acidity, and numbers greater than 7 increasing alkalinity