1) A diverse family of low molecular weight compounds such as cyclic AMP and calcium ions, which transmit the biological signals initiated by receptor-ligand binding at the cell surface to intracellular targets such as gene expression.

2) Systems in which an intracellular signal is generated in response to an intercellular primary messenger such as a hormone or neurotransmitter. They are intermediate signals in cellular processes such as metabolism, secretion, contraction, phototransduction, and cell growth. Examples of second messenger systems are the adenyl cyclase-cyclic AMP system, the phosphatidylinositol diphosphate-inositol triphosphate system, and the cyclic GMP system.

3) A cellular substance (as cyclic AMP) that mediates cell activity by relaying a signal from an extracellular molecule (as of a hormone or neurotransmitter) bound to the cell's surface