Yes that was what my point was aiming at : 'we' usually think login -> purchase as if it is The Way to purchase things online, while I think the login is before all a security measure (or for statistics purpose, or to force newsletter or to gather email address and make profit out of it, there are a lot of reason that can make a company wanting people to create accounts)

And that's what GOG and other members here mean when they say that you "own" the game. You have control instead of the publisher/distributor.

You back it up to whatever media you want. You don't need a client or authentication to install it and play the game. You can do that now or 10 years from now. In this context it makes no difference.

Exactly!

So perhaps it can be said that single player and multiplayer can be different when it comes to DRM.

As for the definition and why it's subjective, the "management" part of the DRM can be seen as an active ability or a passive one. Those who consider passive to be management will see keys as DRM, those who don't will only consider online activation as DRM.

Isn't iTunes DRM-free nowadays? I honestly wouldn't know - I've actually never bought a digitally-downloaded song (I almost never buy CDs either - I listen to the radio).

Not necessarily, although it is true that the primary motivation of many publishers to use Steam - especially with retail products - is its capacity as a DRM provider. If Valve suddenly declared that they were abandoning DRM, Squeenix and the like would immediately up and shift to another platform.

Difficult to say. There are some games that really were designed with a multiplayer experience in mind, and it's hard to say whether the intention to control content post-sale was there.

However, there are almost certainly games out there that are not inherently MMOs, but have been made such to justify their absurd DRM (Sim City, Diablo 3).

Windows has DRM, it isn't DRM.

Songs are, yes, I think audiobooks as well. Not (e-)books or applications, nor do I think videos are DRM-free.

A CD key that is not verified online does not have to be unique. It just has to be valid, because your computer has no way of knowing whether that key is already in use elsewhere.
Yoda of Borg am I. Assimilated you will be. Useless resistance is, hrmmm?

DRM -- prevents you from installing a game installer on your computer offline without requiring an internet connection or cd key or serial or anything else will prevent you from doing that.

DRM means never having to say you're sorry.

What if it has lan or local coop?

Everything that only exists to annoy customers. ;)

On a more accurate and consense basis: Everything that after the initial purchase and initial download depends on the goodwill of the seller.

They'd shift to another DRM provider, but probably won't stop using Steam as a store if it allows that. There are already games with third party DRM on Steam.

The difference is that they would have to pay the DRM provider.

So? I mean, that's true, but how does it matter?