Mesomeric effect is a permanent effect in which $\pi$ electrons are transferred from a

The complete displacement of electron pair caused by an atom or a group along a chain by a conjugative mechanism is called the mesomeric effect of that atom or group. It is possible when some multiple bonds are in conjugation (single and double bonds alternatively) or when an atom withat least one lone pair is in conjugation with multiple bond. It is a permanent effect and it operates untill conjugation ends. Lone pair and  electrons are involved in it