The acid-ionization constant, $K_a$, quantifies the strength of a weak acid in solution. It is the equilibrium constant for the dissociation of an acid into its conjugate base and a proton.
Equilibrium Constant: A numerical value that expresses the ratio of concentrations of products to reactants at equilibrium for a reversible reaction.
$pH$: A measure of hydrogen ion concentration; a logarithmic scale used to specify the acidity or basicity of an aqueous solution.
$pKa$: The negative logarithm of the acid-ionization constant ($-\log(K_a)$); it provides insight into the strength of an acid.