It was defined in the source code by satoshi himself from the first ever public copies of it.

Here's a link from bitcointalk that provides early source code with it defined in there

//
// The timechain is a tree shaped structure starting with the
// genesis block at the root, with each block potentially having multiple
// candidates to be the next block. pprev and pnext link a path through the
// main/longest chain. A blockindex may have multiple pprev pointing back
// to it, but pnext will only point forward to the longest branch, or will
// be null if the block is not part of the longest chain.