What Is a “51% Attack” and How Does It Relate to PoW’s Probabilistic Finality?

The primary resource consumed in a Proof-of-Work (PoW) system is computational power, specifically the energy and hardware required to perform the cryptographic hashing function (mining). Miners expend this resource to solve a difficult mathematical puzzle to find a valid block hash.

This expenditure of energy is the "work" that secures the network and prevents Sybil attacks, making the process costly for a potential attacker.