Batallion or Division

It doesn't matter what you call it, it is a unit. There's no such separate things like a 'battalion' or a 'division'. Its called different for different units. If you create an infantry, it is 1 unit. 1 infantry.

Counting the amount soldiers (manpower is about 1000) it’s the size of a battalion though.

In the game, a single infantry unit is called a Brigade, and a stack is called a Division.

A single ship is a Squadron, and a stack of ships is called a Flotilla.

And so on for artillery and airplanes.

Grouping units of different types can change the name of the stack.

Sometimes the stack inherits the name from its strongest unit (for some definition of strongest).

For example, adding an AA gun to 9 infantry makes an AA division.

This doesn't really tell you much, it's just to make the names more interesting.