Same definition (except I don't think you want marginal cost there, because fixed costs - not just the increase in fixed costs - as well as opportunity costs -

Same definition (except I don't think you want marginal cost there, because fixed costs - not just the increase in fixed costs - as well as opportunity costs - should be included), but of course market power can be used to restrain trade, take over adjacent markets, etc; as well as just soak the consumer in the short run. All these are abuses and illegal - still the litmus test of being able to charge extra is a good one.