Something that gives you pleasure, makes you happy, something that makes you forget your problems for a moment. It may be a movie, game or anything that takes you to the world of fantasy. It can even be a conversation with someone you love. Entertainment often hits on the points that the human mind was evolved to react to deeply. Things like backstabbing, murders and other themes in the social world.
It is often abbreviated to entmt. on fliers or in industry news publication where space is limited. However, it is rarely abbreviated in general writing. More similar words are entertain, divert, amuse, enchant, titillate, diversion, and gratify.