1948, abbreviation of long-playing phonograph record.
The most revolutionary development to hit the recording industry since the invention of the automatic changer is the Long Playing record, which can hold an entire 45-minute symphony or musical-comedy score on a single 12-inch disk. ... The disks, released a few weeks ago by Columbia Records and made of Vinylite, have phenomenally narrow grooves (.003 of an inch). They are played at less than half the speed of the standard old-style records. [Life magazine, July 26, 1948]