1. That which carries a message.
2. Having message-carrying properties.
first messenger, a hormone that binds to a receptor on the surface cell and, in so doing, communicates with intracellular metabolic processes.
second messenger, an intermediary molecule that is generated as a consequence of hormone-receptor interaction; e.g., see adenosine 3′,5′-cyclic monophosphate; guanosine 3′,5′-cyclic monophosphate; calcium; inositide.