Entertaiment means something that affords pleasure, diversion, or amusement: a play or opera is an entertainment. An agreeable occupation for the mind: solving a crossword puzzle is an entertainment. A public amusement: a circus or freak show is an entertainment. The word entertain comes from the Medieval Latin inter tenere, which means to hold inside. It is related to the Indo-European root ten, which means to stretch or extend.
Entertaiment is very often focused on themes that the brain was evolved to react deeply to, wich can be seen in stories about backstabbing, murders and other things the mind was shaped to understand.