[wyldfire]

A Sybil attack is unrelated to consensus, but otherwise what you said is accurate.

A Sybil attack can occur when a system designer mistakes a system identity to be exclusive with other ones. e.g. you could create a website that used a phone number as a user identifier and give every new user a $5 signup bonus. But if someone found a way to create valid phone numbers for less than $5 each, they could exploit your new user bonus system.

If it's free/cheap/easy to create new identities, you must be careful how much weight to give those identities. e.g. reddit has some problems related to how easy it is to manipulate voting. They have probably made changes to their algorithm over the years to deweight new/unverified users and likely other clever things to detect voting rings.

[teod]

Ah ok, I think that a 51% attack is what I was describing. I would update my comment, but it appears that I can no longer edit it.