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

Delocalisation of π-bonded electrons with lone pair electrons which are present on an adjacent atom is called metomeric effect.

Delocalisation of π-bonded electrons to an adjacent single bond is called mesomeric effect.

(or)

Delocalisation of π-bonded electrons from a multiple bond to an adjucent atom is called mesomeric effect