exogen

This term is primarily used in specialized scientific contexts, particularly in biochemistry, pharmacology, and cellular biology. It describes any external agent introduced into a biological system to trigger a specific reaction or maintain a function, contrasting sharply with an endogen, which is produced internally by the organism.

Because it is a technical term of art, it is rarely encountered in general conversation and is almost exclusively found in peer-reviewed literature or laboratory reports. It carries a neutral, descriptive tone and focuses on the origin of the substance rather than its specific chemical identity.