UK law basically doesn't use the term.

My point was that proving dominance and abuse is rarely objective fact. It sure isn't showing market share and that some games companies go out of business. They have to show the things that valve does to restrict competition - being popular isn't enough alone.

Your last question is quite a good example of how hard it is to prove because it includes counterfactual comparisons.

This might be why it seems (if the journo is to be believed) that they're going down the tie-ins angle for the DLC, not necessarily headline pricing. Thou the latter would probably a worse outcome for valve if guilty.