Out-lap
An out-lap is the lap immediately after leaving the pits, driven on cold tyres and brakes; the in-lap is the lap before pitting.
Also called in-lap
The out-lap decides whether an undercut works. Cold rubber has a fraction of its grip, and a driver has one lap to bring it into range while going as fast as they dare — too cautious and the advantage is gone, too aggressive and the tyre grains.
The in-lap matters for the same reason in reverse: every tenth found on the way in is a tenth the stop does not cost.