Entertaiment is anything that amuses, distracts or delights. The clown at a birthday party, the Broadway show, your friends fighting over the last potato chip — these are all forms of entertainment. The word derives from the Latin entretenere, meaning support or hold together, originally associated with hospitality.
The most successful entertainers often hit on themes that the brain was shaped to respond deeply to. Murders, backstabbing and social cruelty are popular themes for entertainment because the brain was shaped to understand these things. These types of entertainment stimulate the brain to release seratonin and dopamine. These chemicals give a good feeling, which is why we like to get entertained.