four-stroke engine: an internal-combustion engine in which an explosive mixture is drawn into the cylinder on the first stroke and is compressed and ignited on the second stroke; work is done on the third stroke and the products of combustion are exhausted on the fourth stroke
entity: that which is perceived or known or inferred to have its own distinct existence (living or nonliving)
endorphin: a neurochemical occurring naturally in the brain and having analgesic properties