from page 31 Path of War:
"Counter: A counter is a fast, usually defensive maneuver that is used to prevent or respond to your opponent’s actions. A martial disciple who dodges just beyond an ogre’s reach is using a counter."
[cut some irrelevant text about iron tortoise using shields here.]
"Counters are immediate actions that you attempt during an enemy’s turn. Usually, your foe must make a specific action, such as an attack against you, for you to use a counter."
also, same page:
"CRITICAL HIT VS COUNTER
As this is bound to come up at the gaming table at least once in a while, a player or the DM will roll a natural 20 against a creature or player and the decision to use a counter will arise. Relax, here's the answer. Counters may be used against a critical hit except if the threatening attack roll was a natural 20. Critical hits from weapons with extended threat ranges (such as rapiers) can be countered if the threatening attack roll was not a 20."
that side bar is what makes me answer the way I do.