Killer is a versatile English word with multiple meanings depending on context.
1. Literal meaning: A person or thing that kills (murderer, assassin, etc.)
2. Slang meaning: Something extremely good or impressive ("That concert was killer!")
3. Informal usage: Something very difficult or demanding ("That exam was a killer")
4. In sports: A decisive move or play ("He delivered the killer blow in the final round")
5. In business: A product or feature that dominates the market ("The new iPhone is a killer app")