A decentralized identifier: an ID you control instead of one you're assigned
A decentralized identifier is a globally unique identifier that a user generates and controls themselves, rooted in their own cryptographic keys, rather than one handed out and administered by a centralized registrar like a government agency or a platform's account system. Where a traditional username or account number only means anything because some central authority's database says it does, a decentralized identifier is self-certifying: control over it is demonstrated by controlling the corresponding private key, the same underlying idea as the wallets covered elsewhere on this site. No company can delete your identifier, freeze it, or reassign it out from under you, because no company issued it in the first place.
This matters because it flips the starting point of the whole system. Instead of asking 'which central authority recognizes this identity,' the question becomes 'can this identifier prove control of itself through cryptography.' That shift is what allows the rest of the decentralized identity model to exist without a central operator, an identifier is portable across contexts and organizations because its validity was never dependent on any one of them to begin with.