Pseudo first order is a condition under which the order of a chemical reaction can be altered by taking the solvent or the reactant in excess amount due to which its concentration does not change much.

For example,

CH3COOCH3 + H20 ⇌ CH3COOH + CH3OH

Initially rate is given by,

Rate= k’ [CH3COOH] [H2O]

When we add excess of solvent then,

Rate= k[CH3COOCH3]

Here k= k’[H2O]

This is also known as pseudo first order reaction.