Selective Serotonin Reuptake Inhibitors refers to a drug that blocks the reuptake of Serotonin into the presynaptic terminal.

Selective Serotonin Reuptake Inhibitors is also calle d SSRI.
Other /More definition:
Selective Serotonin Reuptake Inhibitors (SSRI) refer to medications that block the reuptake of serotonin at the synapse, enabling more of this neurotransmitter to be available at the receptor sites.