Physiology. An intracellular mediator that is generated in response to an intercellular primary messenger such as a hormone or neurotransmitter. The hormone does not enter the target cell but binds to a membrane receptor on the cell surface and indirectly affects the production of another molecule within the cell. This diffuses intracellularly to the target enzymes or intracellular receptor to produce the response. Second messengers intermediate signals in cellular processes such as metabolism, secretion, contraction, and cell growth. Examples include cyclic AMP, cyclic GMP, IP3 and diacylglycerol.