reuptake inhibitor
A reuptake inhibitor is a drug that makes a neurotransmitter's message last longer by jamming the recycling pump that would normally clear it away. Imagine the sending neuron releases its messenger and then quickly vacuums most of it back up; a reuptake inhibitor blocks that vacuum, so the messenger lingers and keeps stimulating the receiving cell.
Mechanistically, after a neurotransmitter is released, much of it is reabsorbed into the presynaptic neuron through a specific membrane transporter protein. By blocking that transporter, a reuptake inhibitor raises the concentration of neurotransmitter in the synaptic cleft and prolongs its action, without itself activating any receptor.
This is one of the most heavily exploited mechanisms in CNS pharmacology. SSRIs block the serotonin transporter; SNRIs block serotonin and norepinephrine transporters; bupropion blocks dopamine and norepinephrine reuptake; and stimulants like cocaine and amphetamines act largely through dopamine transporter blockade.
A subtle point: blocking reuptake only amplifies whatever signal is already being released, so the effect depends on ongoing neurotransmitter release. It is also why the precise transporter selectivity of a drug shapes both its therapeutic use and its side-effect and abuse profile.
An SSRI is a reuptake inhibitor with narrow selectivity for the serotonin transporter, which is what distinguishes it from broader inhibitors that also block norepinephrine or dopamine reuptake.
Transporter selectivity defines which reuptake inhibitor you get.
Reuptake inhibition raises an existing signal, whereas an MAO inhibitor raises it by blocking breakdown — two different ways to get more neurotransmitter, which is exactly why combining them can dangerously overshoot.