Army Area (自軍エリア Jigun Eria) is the official term for the 3 Squares closest to your position (your own Player Square and two Squares on its left and right). It is the opposite of Enemy Army Area.
This term is used by cards that only affect the Z/X in those particular Squares, all of which are White cards. From Set 5 onward, the usage of this term fell in favor of Square Designation Diagram.
List of cards that mentions "Army Area" in its text