Answer: An autoreceptor is a receptor for a neurotransmitter that is expressed on the same neuron that releases the neurotransmitter.
When a neuron releases a neurotransmitter, the neurotransmitter molecules follow the rules of Brownian motion. The molecules are released into the synapse, the gap between two neurons. There, the neurotransmitter can travel across the synapse and activate transmembrane proteins on the postsynaptic cell. These proteins are the receptors.
There are often also receptors expressed on the cell that just released the neurotransmitter. These receptors respond to the same neurotransmitter that was released, and these are the autoreceptors. Most autoreceptors are inhibitory, that is, they decrease the likelihood of neurotransmitter release. Therefore, they prevent the neuron from releasing too much neurotransmitter. This self-inhibition system is called negative feedback. Most neurotransmitter systems have some sort of autoreceptor function.
Autoreceptors are generally G-protein coupled receptors rather than ionotropic receptors. They affect the cell through activation of second messenger transduction.
The dopamine D2 receptor, norepinephrine alpha 2a and alpha 2c adrenoreceptors, acetylcholine M2 and M4 muscarinic receptors, and histamine H3 receptors are examples of common autoreceptors expressed in the nervous system.