Very traditionally speaking, metagaming is when a character uses knowledge that the player has, but of which the character should be unaware. Examples of metagaming include using fire spells against a troll (when the character has no knowledge of trolls), or specifically checking behind a certain pillar for a hidden treasure cache (which the player knows about from having read the module).
Likewise, if the player knows how to bypass a trap (either from having read the module, guessing how the DM plots, or personal understanding of how machines work), then the character does not necessarily possess that knowledge. The first two points are obviously just cheating. The fair and established way to determine what the character does or does not know, regarding that third point, is to roll for it. Instead of the player saying that she has her character position a shield in front of the firing mechanism in order to render the trap harmless, you roll to determine if the character can figure that out; and if he can, then the DM is free to describe how he bypasses the trap.
It makes no sense for an ugly barbarian to fast-talk a guard just because the barbarian's player is a stand-up comedian, when the barbarian has low charisma and no proficiency. If the player uses player knowledge about how to talk to people, instead of relying on character skill (as governed by the die roll), then that's pure meta-gaming. It's the opposite of role-playing.