PH

From Distillers Wiki

A chemical shorthand for [p]otential of [H]ydrogen, used to express relative acidity or alkalinity in solution, in terms of strength rather than amount, on a logarithmic scale. A pH of 7 is neutral; above 7 is increasing alkalinity and below 7 is increasing acidity. Thus, a pH of 3 is 10 times more acidic than a ph of 4. See Acidity.

Effectively this is a measure of the availability of nutrients in an aqueous solution. pH is an important consideration for any wash.