Which term best describes a neurotransmitter that inhibits the action of another neurotransmitter?

The term that best describes a neurotransmitter that inhibits the action of another neurotransmitter is "antagonist." Antagonists are substances that bind to receptors in the nervous system and block or inhibit the effects of other neurotransmitters, effectively reducing or counteracting their action. This means that when an antagonist is present, it prevents the typical signaling processes associated with the neurotransmitter it inhibits, leading to a decreased response in the target cells.

In contrast, an agonist would enhance or mimic the action of a neurotransmitter, facilitating or increasing the intended signal rather than inhibiting it. Other terms like "enhancer" and "facilitator" are less common in the context of neurotransmitter action and do not specifically denote the inhibiting effect characteristic of antagonists. Therefore, "antagonist" is accurately associated with the role of inhibiting neurotransmitter functions.