Definition of villain
At a poker table, any player who is not us (Hero) is considered a villain (naughty in English). When we speak more specifically of villain, we will be talking about a player that we have faced in a specific hand. When you want to ask friends for advice on a hand you have played, you will present the hand in question and the player whose cards are seen (you) will be called "hero" in English or héro in French, and your opponent will be categorized as a villain.
Reversing the roles
The roles can also be reversed for a reverse hand analysis. For example, let's say you bluffed on the river and you want to know if the bluff is good. You could reverse the roles and pretend that the hero is the villain to see if you bluffed right. You could then ask your friends, whether or not hero (who is actually villain) should call the bet of villain (who is actually hero). This is a common vocabulary to help you understand each other when you see hands again.