A weak acid is an acid that does not fully ionize in solution; that is, if the acid was represented by the general formula AH, then in aqueous solution a significant amount of undissolved AH still remains.
HA(aq) 
H+(aq) + A-(aq)
The equilibrium concentrations of reactants and products are related by the Acidity constant expression, (Ka): Ka = [H+][A-]/[HA].
The greater the value of Ka, the more the formation of H+ is favored, and the lower the pH of the solution.
The vast majority of acids are weak acids.
Examples