A fencing term for attacking into an already declared attack.

Imagine an opponent rushing head on at you with all of their might... At the final moment, rather than defending yourself, you take at swipe at him/her too. This is the usual example for what is referred to as a counterattack.

99% of the time it's a mistake, but it's almost always a very tempting mistake to make, especially as a beginner learning right of way.