"Sipa" definition? (old terminology, circa 2011-12)
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.