Vanitygen refers to "Sipa" format here:
Currently, it is difficult to import the private key into bitcoin.
Sipa's showwallet branch has a new command called "importprivkey" that accepts the base-58 encoded private key. Vanitygen has been tested to work with that version of bitcoin.
I remember this term being thrown around when I was relatively new to Bitcoin, probably around the same time as the Vanitygen quote was made. There's a conflicting references to sipa format, it seems, for example, Multibit uses Sipa in place of WIF, whereas the aforementioned quote makes no sense in that context. BitcoinTalk seems to suggest Sipa = Base58 WIF
What, exactly, does the term
Sipa refer to? (EDIT: And where did the term come from?)