Entertainment is amusement, usually involving a performance. The clown at a birthday party, a Broadway show, a stadium rock concert, your friends fighting over the last potato chip––these are all forms of entertainment.
Entertainment is from the Old French word entretenir meaning hold together or support. It was associated with hospitality––when you entertained a guest, you were keeping them happy. From there, it came to mean amuse or distract. The entertainment is the amusement or the distraction––we talk about theatrical entertainment as shows, visual entertainment as the stuff you like to look at, and audio entertainment as mostly music.