Formal definition of fortresses

[Stein]

Hi,

Does there exist a formal definition of fortresses?

As can be seen in: https://en.wikipedia.org/wiki/Fortress_(chess) and https://www.chessprogramming.org/Fortress there are many types of fortresses.
Wikipedia takes a step towards defining fortresses in an informal manner:

Fortresses commonly have four characteristics:

Useful pawn breakthroughs are not possible.
If the stronger side has pawns, they are firmly blocked.
The stronger side's king cannot penetrate, either because it is cut off or near the edge of the board.
Zugzwang positions cannot be forced, because the defender has waiting moves available (de la Villa 2008:23).

The question is: has there been an attempt to define fortresses formally?

Has there been an attempt to define a subset of fortresses formally (there are several types of fortresses)?

Thanks,
Stein

[hgm]

In EGTs I classify any position where a draw can be acheived without perpetual checking or forced material gain as a fortress draw. Perpetual checking is really a subset of fortress draws, but it deserves to be classified separately.