A “Sybil Attack” is a form of cyber-crime whereby a malicious party takes advantage of an open systems with pseudonymous/anonymous properties (identity/reputation parameter) and floods the system with a large number of false identities. The fake identities in turn give the malicious actor control over the social aspect of a network.
Sybil Attack = multiple identities for single entity
In the context of cryptocurrency and blockchain, sybil attacks are used on networks that are heavily reliant on decentralized governance. Because of their pseudonymous nature, malicious actors instantiate huge amounts of accounts which they then use to promote their thoughts and ideas. This subverts the natural progression that is to take place and is instead replaced by a single parties desires.