mid-15c., "occupying time, made longer," past-participle adjective from extend (v.). Meaning "stretched out" in space is from 1550s; extended-play (adj.), in reference to recordings (especially 7-inch, 45 rpm vinyl records) is from 1953; in reference to pinball games by 1943. Extended family (n.) in sociology recorded from 1942.
A challenging question was asked RCA engineers and scientists in 1951. How can we increase the playing time of a 7-inch record, without using a larger disc? Sixteen months of research gave the answer, "45 EP"--Extended Play. [Radio Corporation of America magazine advertisement, May 1953]