Second messenger:

substance released after the activation of a receptor by a "first messenger" like a neurotransmitter, hormone, etc. Second messengers may serve to amplify or prolong the effects of a first messenger, initiate signaling cascades in the cell, or have other effects. Well-known second messengers include cyclic adenosine monophosphate (cAMP), diacylglycerol, and arachidonic acid.