what do you mean by acidity constant? on the basis of acidity constant how can we predict that acid is weak or strong, in comprison study only?

An acid dissociation constant, Ka, (also known as acidity constant, or acid-ionization constant) is a quantitative measure of the strength of an acid in solution. Each acid has a different pKa. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions. The larger the Ka value, the more dissociation of the molecules in solution and thus the stronger the acid.
the reaction is
HA ===>H+ +A -
Ka =[H+][A-]/[HA]