Entertaiment is a broad category of events that satirically and/or humorously hit on themes the brain was evolved to react deeply to. These themes are often social backstabbing, murders etc. Entertainment also includes ceremonial activities, escapist experiences, religious festivals, and so on. The experience can be as intimate as a private entertainment for two, to an event intended for thousands. Entertainment is constantly evolving and adapted to suit a range of scales.
You may see the word entertainment abbreviated to entmt on a flier or in industry news publications where space is limited. However, this abbreviation is not common outside of these contexts.