Proof of Work
The consensus mechanism Bitcoin uses to add new blocks. Miners compete to solve a computational puzzle that requires enormous effort but is trivially easy to verify. This asymmetry is what makes attacks on Bitcoin's history prohibitively expensive.
Proof of Work is the consensus mechanism Bitcoin uses to agree on the state of the blockchain without a central authority. To add a new block, a miner must solve a computational puzzle: find a number (the nonce) that, combined with the block's data and hashed with SHA-256, produces an output below a target value. This requires billions of trial-and-error attempts and enormous computational effort, but the solution can be verified by any node in less than a second.
This design creates a powerful economic security property. To rewrite Bitcoin's transaction history, an attacker would need to redo all the proof of work for the targeted block and every block that followed, faster than the honest network adds new blocks. The cost of such an attack scales with the total hashrate of the network — as more miners participate and the network grows stronger, the attack becomes more expensive. This is why Bitcoin's security increases over time as adoption and hashrate grow.
Proof of Work does require significant energy consumption, which is regularly discussed. Miners have economic incentives to seek the cheapest electricity available, which increasingly comes from stranded or renewable energy sources. The energy expenditure is the intentional cost that makes the system trustworthy: every block represents real-world resource expenditure that cannot be faked or undone.