Alien Year Zero was the in-development title for Alien: Isolation.
The developers, The Creative Assembly, were believed to have officially started hiring for the project in 2011 after beginning early work in 2008. The game released in 2014 on Xbox 360, Xbox One, PS3, PS4 and PC. It was ported to MacOS and Linux by Feral Interactive in 2015.
Alien Year Zero (or AYZ) was a title seen most publicly from employees of Axis Animation while developing the CG Trailer for Alien: Isolation, and can also be found within the Steam Database for the depot and package names of the game. The beta of Alien: Isolation is named "AYZ Beta" and the early access package of the game is named "AYZ Early Access Comp".
It's not known how long the name stuck for, why it was changed, or if it was just a working title to keep the official title secret. Some game files such as animations typically begin with the prefix of "AYZ" and a number of people involved in the game refer to it as "Alien Year Zero" in their online portfolios so it is possible that the name was kept for a large period of the development cycle, or used for communicating about the project to people outside of The Creative Assembly.
The trademark for "Alien: Isolation" was filed on the 17th of October 2013, with no trademark being filed for "Alien Year Zero". The domain "alienisolation.com" was registered on the 6th of February 2013, showing that the title was in-use at least around a year before the game's announcement.