What is pseudo first order reaction. Why is it used?

This is a second-order reaction in which one of the reactants is present in such great amounts that its effect is not seen. For example, the hydrolysis of an ester in water. If the water is present in great amounts, any change will be too small, relatively, to be noticed. The reaction then appears dependent on concentration of ester alone. To see the dependence on water, you'd have to dissolve the ester in acetone with a few % water.