How Does a ‘Sybil Attack’ Differ from a 51% Attack in a Blockchain Context?

A Sybil attack is an attempt to gain disproportionate influence on a network by creating numerous false identities (nodes or users). In PoW, a Sybil attack on mining nodes is ineffective due to the cost of hash power.

In PoS, it's mitigated by requiring a stake per validator. A 51% attack is specifically about controlling the majority of the network's resource (hash rate or stake) to manipulate the ledger, whereas a Sybil attack is about overwhelming the network's identity count.