Powergaming and Metagaming
It's not cool.
What is powergaming? Powergaming is a pose where you pose the results of another character's interaction over with yourself or in a scene without their permission. It can also be putting someone in circumstances where they have no options for response except a coercive, specific action.
Examples: Thing's player emitting, 'Thing hits Abomination in the face, and knocks him down with one punch!' Or: Quicksilver's writer stating 'Quicksilver is so fast, that no one can touch him this round and no one can remotely land a shot on him'. This is *technically* within his powerset, but not conducive over to the scene. It states that *any* action taken against him will fail, pre-emptively.
Powergaming can also be as a result of posing powers (such as pheromones, mind control) or circumstances no one can possibly resist or react to (someone with superspeed, grabbing someone so they can't move) or circumstances where there is no option and participation or doing things in such a manner is enforced (abusing IC authority by humiliating someone or giving them unreasonable orders, having someone completely resistant to all effects to stop them or invulnerable to things, having characters perform unreasonable actions in circumstances or not giving other characters attempts to stop them or interrupt, etc).
With everything, it's always best to pose your attempt and then let someone else pose the result, or if you want to do something to another character or setup someone is emitting, to ask them first. As always, if unsure: ask! Pose as an attempt then without the consent of another player, and it's helpful in such things to let other participants in the scene know quickly so as to not interrupt the flow of things.
The easy definition of Metagaming is using OOC knowledge for IC gain when the character would otherwise not have that information. This is when you as a player know something (ie, Bruce Wayne is Batman) and then you roleplay your character as having that information.
Metagaming is also knowledge of cutscenes or logs that your character was not ICly present for, or knowledge that is given in a pose but not outright said (metaposing vs narration). A writer might emit their character lying through their teeth, but unless your *character* knows that they're being lied to, you shouldn't simply leap to portraying distrust. Even someone like Jean Grey can and has been lied to canonically.