The effect (on reaction rates, @I03183@ equilibria, etc.) attributed to a substituent due to overlap of its p- or π-orbitals with the p- or π-orbitals of the rest of the @M03986@. @D01583@ is thereby introduced or extended, and electronic charge may flow to or from the substituent. The effect is symbolized by M. Strictly understood, the mesomeric effect operates in the ground @ET07026@ of the molecule. When the molecule undergoes electronic excitation or its energy is increased on the way to the @T06468@ of a @C01033@, the mesomeric effect may be enhanced by the @E01973@, but this term is not much used, and the mesomeric and electromeric effects tend to be subsumed in the term resonance effect of a @S06076@.