opiate. 1  a drug containing opium or its derivatives, used in medicine for inducing sleep and relieving pain. 2  any sedative, soporific, or narcotic. 3  anything that causes dullness or inaction or that soothes the feelings. 4  mixed or prepared with opium. 5  inducing sleep; soporific; narcotic. 6  causing dullness or inaction. 7  to subject to an opiate; stupefy.