Definition of ENDORPHIN. : any of a group of endogenous peptides (as enkephalin) found especially in the brain that bind chiefly to opiate receptors and produce some pharmacological effects (as pain relief) like those of opiates; specifically: beta-endorphin.ADVERTISEMENT. any of a group of endogenous peptides (as enkephalin) found especially in the brain that bind chiefly to opiate receptors and produce some pharmacological effects (as pain relief) like those of opiates; specifically: beta-endorphin. ADVERTISEMENT.