The Collaborative International Dictionary of English v.0.48:

long-playing \long-playing\ adj.
   playing at a slower speed and for a longer time than earlier
   records; -- used of phonograph records.

   Syn: long-play, LP.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

long-playing
    adj 1: (used of records) playing at a slower speed and for a
           longer time than earlier records [syn: long-play,
           long-playing]