The term "pKa" is a measure of the strength of an acid in solution. It is defined as the negative base 10 log of the acid dissociation constant. A lower pKa value indicates a stronger acid.
Dissociation is the reaction by which compounds split, usually reversibly, into atoms or ions. The acid dissociation constant is a measure of an acid's tendency to dissociate in water. A strong acid has a high tendency to give up its hydrogen ion while in solution, while a weak acid is more likely to hold on to its hydrogen. This means that strong acids dissociate more easily in water and have a higher dissociation constant. Weak acids are less likely to dissociate.
Acid dissociation constants cover many orders of magnitude. The negative base 10 logarithm provides an easier way to report dissociation. The pKa and the acid dissociation constant are inversely related to one another; low pKa values point to stronger acids, while high pKa values indicate weak acids.
The acid dissociation constant and pKa are different measurements than pH, another common measurement of the strength of acids. Where pKa deals specifically with the strength of acids, pH deals with acidity or alkalinity of a solution. Strong acid in water often has a rather low pH, but weak acid at the same concentration might have a pH much closer to neutral.