How Does a 51% Attack Differ from Selfish Mining in Terms of Goals and Impact?

A 51% attack requires a majority of the hashrate to execute malicious actions like double-spending or preventing transactions, aiming for systemic disruption or theft. Selfish mining, however, requires only slightly over 25% of the hashrate and is a profit-maximization strategy for the attacker by gaining an outsized share of block rewards.

The 51% attack targets network integrity; selfish mining targets reward distribution efficiency. Both exploit the PoW consensus mechanism.