pH means Potential of Hydrogen,Power of Hydrogen or Hydrogen ion concentration.
Thus it's defined as the negative logarithm to base 10 of Hydrogen ion concentration. This definition of pH was devised by a man called Sorensen in 1909.
pH is measured using a pH meter,but the acidity and alkalinity of a substances are measured with a pH scale.
pH scale is calibrated from 0 - 14.
It is observed that a solution with a pH value of 7 is neutral,ie it's neither acidic or alkaline. A solution with a pH value less that 7 is acidic,while a solution with a pH value greater than 7 is alkaline.
Acidity increases with decrease in pH values whereas alkalinity increases with increase in pH value.