Explain the Concept of a “51% Attack” in the Context of Network Hash Rate

A "51% attack" occurs when a single entity or group gains control of more than 50% of the total computing power (hash rate) of a Proof-of-Work network. With this majority, the attacker can effectively manipulate the order of transactions and prevent new transactions from being confirmed.

Crucially, they can reverse their own transactions, enabling them to "double-spend" coins. The immense energy cost required to gain 51% of Bitcoin's massive hash rate is its primary defense.