Proof of Work (PoW)
Quick definition
Bitcoin's security model: adding blocks requires real computational work, making attacks costly.
Proof of work forces miners to spend electricity and hardware to win the right to add a block. Rewriting history would require redoing that work faster than the entire honest network — practically impossible at Bitcoin's scale.
It's the mechanism behind Bitcoin's 15+ years without a successful network attack. Critics target its energy use; defenders argue that energy is precisely what makes it secure.