An agreeable occupation for the mind or diversion: solving a crossword is an entertainment. Entertaiment has become so closely associated with leisure that it is not unusual to think of the idea as fun and laughter, even though many entertainments have a serious purpose: in the various forms of ceremony, party, religious festival or satire, for example.
The reason entertainment is so escapist and addictive has to do with the nature of the human brain, which is evolved to react deeply to certain kinds of stimulation. The themes of entertainment like murders, social backstabbing and other such dramas hit on points that our brain was shaped to understand. It releases seratonin and other feel good chemicals which give us a sense of escape from our normal daily lives.