That is the North Atlantic Track Z, or track Z / NATZ for short.
NAT’s are routings across the north atlantic ocean that are updated daily, to facilitate aircraft flying “across the pond” with the most favorable winds on their journey. This also makes it easy for ATC to seperate using either longitudinal, latitudinal seperation (but that’s advanced).
Despite being updated daily, they share the same lettering. Westbound NAT tracks start with the letter A and forward, and eastbound tracks start with the letter Z and go backwards.
To see the current NAT tracks, go to this website. :)
If you’ve got any more questions about NAT tracks, or oceanic procedures tag and ask me!