Powered: Any mechanic that affects game-play. For example: copper tools and armor, electricity system, mobs that drop loot, barrels. Not an example: new colors of wool, bats, name tags, complex models, hats.
Overpowered: Any powered mechanic,that allows faster and more productive game-play (better) than regular game-play. For example: double ore gain (macerators, pulverizers, etc), horses (faster travel), coal to diamond recipe, super-powerful armors, cheaper diamond level protection armor, double gain circuit crafting recipes, any automation of mining, breeding, or farming, cheaper rail recipes, more powerful lava generator, cobblestone*.
Underpowered: Any powered mechanic, which is more costly and less useful (worse) than regular game-play. For example: Macrerators compared with pulverizers, bricks compared to cobblestone, GT automation compare to all other automations, arguably factorization ore gain tripling (due to the ease of mining 50% more over having the extremely complicated and expensive setup).
Not to be confused with:
Game-breaker: Any mechanic, or combination of mechanics, that utterly nullifies the intended game-play completely and thus break the game. For example: dirt to diamond, Endgame of most mods for other mods, philosopher stone for other mods, automated mining, farming, and breeding for vanilla, GT and anything for either GT or everything.
*cobblestone is a very very cheap and easy to get block that has high blast resistance, making it by far the best building material