PvP and PvE
Also said as Player versus player, Player versus environment
Whether you are playing against other people, or against the game. They change more than the opponent: a script can be learned, and a person learns you back.
The same ability gets tuned differently for each, because what is fun to do to a boss is often miserable to have done to you. Mixing them without thought is how a PvE player ends up as someone else's entertainment.
Say it
“Separate PvP and PvE numbers for every ability, so that we can halve a stun in PvP without touching raids. Players opt into PvP in town and cannot be attacked otherwise.”
Where you’ve seen it
World of Warcraft used to split its realms into PvP and PvE, and Battle for Azeroth replaced that split with War Mode, a toggle letting each player choose whether they can be attacked.
Not to be confused with co-op
PvE is about the opponent, the game. Co-op is about who is beside you. Most co-op is PvE, and team-based PvP has teammates too.