Self-Sovereign Identity. SSI refers to identity users have complete control over. They can issue, store and permit/revoke access to credentials rather than trusting centralized entities. SSI has a lot of overlap with blockchain, but they exist outside of one another.
There is some debate on the degree to which SSI 'needs' blockchain and vice versa (there are plenty of web2 SSI products emerging from Okta, Microsoft, and startups)
The main aspect blockchain brings to SSI is a trustless, censorship resistant way to store decentralized identifiers (DIDs). The tradeoffs associated w non-blockchain DID warehousing are the meat of the debate on how intertwined blockchain and SSI are/should be.
SSI and blockchain are both young, unestablished, and have struggled with the use case problem, so im hoping both lean into the relationship as much as possible
i'll touch on this more soon!