Edited;
I see my definition can't fit in the submission box so here it is:
Powergaming is defined by a scenario where your or another character is suddenly carrying out an unrealistic or unfair action. With unrealistic is meant that the character performing it, shouldn’t have it or didn’t have it before the scenario started. Things like “Hey this castle wall is in front of us, ‘Don’t worry’ said the dwarf, ‘I’ll just jump over it with my super strong jumping strength’”. Essentially lore breaking things.