Day-one patch
Also said as Day-zero patch, Launch patch
An update that is ready when a game launches, holding the fixes made in the weeks after the final build went off to be manufactured or certified.
A game goes on being fixed after the build is frozen, and the patch is how that work reaches players. The catch is that the disc alone remains the old game, for anybody offline and for ever.
Say it
“Gold is six weeks before launch, so plan for a day-one patch and test the game both with it and without it, since disc-only players will get that version.”
Where you’ve seen it
Halo: The Master Chief Collection launched in 2014 with a day-one patch of about 20 GB, and its matchmaking was still badly broken at launch. A patch is no guarantee.
Not to be confused with unfinished game
A day-one patch usually shows that a gap exists between gold and launch, and the team used it. It does not by itself mean the game was unfinished, though it can hide that.