What is 'ADVENT' an abbreviation for?

Solution 1:

ADVENT is a word meaning approach or arrival. It also has significance in Christian cultures, where Advent is the season before Christmas, where followers anticipate the coming of Christ.

As the plot of XCOM 2 unfolds, the Commander discovers that ADVENT forces are working towards...

...the completion of the Avatar Project.

The goal of this is to...

...create a new series of bodies for the Elders that are far more healthy than their decaying Ethereal ones. (link - beware further spoilers)

I suspect the name ADVENT was deliberately chosen with this in mind, as...

...it reflects both the impending arrival of the Elders to Earth, and heralds their new Avatar forms whose advanced psionic powers could be viewed as godlike.