Threshold
The point at which your dog stops being able to think — past it, nothing you say gets through.
Also called: over threshold · under threshold · threshold distance
Every dog has a distance, or a volume, or a number of dogs in the lift lobby, at which it can still notice something and stay in one piece. Inside that line it can hear you, take a treat, and choose what to do. Past it the thinking part of the brain hands over to the reacting part, and the barking or lunging that follows is not disobedience — it is a dog that has run out of room.
This is the single most useful idea an owner can take away, because it reframes the whole problem. "He knows the command, he's just ignoring me" is almost always "he is past the point where the command is available to him". You cannot train a dog that is over threshold; you can only get it back under and start there.
In practice a trainer will find your dog's line — the distance at which it notices the other dog but does not explode — and work just inside it, then move the line. That is why sessions happen on your street rather than in a hall: the line is different in every environment.