"Sipa" definition? (old terminology, circa 2011-12) 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?)
Sipa is the alias of Pieter Wuille, a Bitcoin Core developer. It just happens that he had a feature branch for the rpc command importprivkey, and people just referred to it as the "sipa" format. It's just something that has hung around in vanitygen and a few places on the Bitcointalk forum, it's not in common use anywhere else. The format being talked about is just base58check encoding with a version prefix differentiating compressed and uncompressed point usage. 

This format is today most commonly known as WIF (Wallet Import Format), and also the format used in the MultiBit classic export.