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](https://bitcointalk.org/index.php?topic=382374.0)

> //
// 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.

nullama

bitcoin

(paying 2100 sats for a good answer!)
I can't find where the term "Genesis" as referred to Block 0 was used for the first time.

Was it Satoshi who named it this way, or bitcointalk forum eventually established the meme. But someone was first - WHO?