The only real official definition is : "We don't believe in controlling you and your games. Here, you won't be locked out of titles you paid for, or constantly asked to prove you own them - this is DRM-free gaming."
https://www.gog.com/about_gog

Of course, life is often a little more complicated with stuff like "not DRM" anti-cheat mechanics being able to lock you out of offline modding of offline titles you paid for. Or being able to play offline but stuff that could easily be locally offline implemented instead being "gated" online as 'bonus content' (see the No Man's Sky thread). Hence why the never-ending debate over what is DRM boils down to ideological rigid definitions "online account ownership verification check only" vs a more inclusive "Do I have 100% offline control of 100% of this game without any artificial restrictions" not always meaning the same thing, when some of this stuff has "dual-use" overlapping functionality / intentions. (See the Steamworks DRM developer page which recommends developers add online functionality as 'functional' secondary "Not DRM" DRM...)

My definition of DRM is the only right one.

Strictly speaking some GOG games have DRM, at least in their multiplayer part. But I personally don't care much about that (=multiplayer DRM), in fact I feel in many cases it is positive to me. Except, I admit it would be nice you could have DEFCON LAN play without having to validate your only multiplayer key online...

My definition of Digital Rights Management (DRM) free or 100% Digital Rights Management (DRM) free is absolutely no email account or any client software launchers required to play a video game you purchase like Steam, Electronic Arts (EA's) Origin, Ubisoft's Connect (previously Uplay), Rockstars Social Club, Epic Games launcher, Denuvo, Blizzard.net's battle.net, ZaniMax Media Inc's and Bethesda Softworks (Now owned by MicroSoft) bethesda.net, and MicroSoft's own MicroSoft/ Xbox Live things you need to make and sign into just to play their video games.

This includes multiplayer as well, there are video games that have 100% Digital Rights Management (DRM) free multiplayer that do not require you to sign into a email account, like gog.com or Steam or type any CD Keys. These video games with 100% Digital Rights Management (DRM) free multiplayer are very rare sadly I can think of three of them right now and those three video games are Unreal Tournament, Red Faction, and Unreal Tournament 2004.

Well this quote

"GOG.COM remains a DRM-free store. We are committed to bringing you more exceptional AAAs, great indies and all-time classics free of any DRM or mandatory clients."

Is actually a blatant lie right now. A long time ago GOG explained that their definition of "DRM-free" only means DRM-free single player. But even that is not true anymore: GWENT of cousre can only be played with Galaxy and then there are those (admittedly few) entries which even lock out some single player content (No Man's Sky Being the latest example, but I already experienced this with Hybrid Wars in which they blocked an additional character before you once connected to their servers ... which funny enough are now down ... and the game removed from GOG).

Luckily for me I prefer SP anyway so for me it is DRM-free (enough) if I am able to install, play and safe SP without being online or inserting any disc ... however I would like to access all SP content that's available so yes, Hyprid Wars was a bit of a letdown in this regard.

How about being able to run the game in a debugger or mod it without having some anti-tamper mechanism tell you you've something they don't want you to do?

Since I usually don't do this I don't care. Like I said, what I mentioned is for ME enough DRM-free, does not mean that it is DRM-free per definition.

So you don't care whether people are able to patch and mod old games for you. That is quite disappointing.

Well I am takjing about things that are important to me and since I normally don't use mods I can live with not having them. Can't see how this purely subjective preference could disappoint anyone.

So if a game doesn't run on your OS / doesn't support your widescreen / has graphics glitches on your GPU / crashes / corrupts saves randomly / etcetra.. you just don't play it?

You mind it if an authentication mechanism makes you unable to play a game, but you don't mind it if a bug makes you unable to enjoy a game?

Sure that's purely subjective, I just find it very bizarre. Usually people who care about and campaign for their digital rights include the right to repair (or run on an alternative operating system..) among those rights.

Usually I won't buy a game that does not run on my OS. I can live with it not having WS but often GOG already implements such features. Let's be clear, we are talking about very few games which needs to be modded at all to be able to run and from those even fewer will have anything in them that will prevent modders to change it. So we are talking about an extremely small number of games that have this problem at all. So I don't think it is worth to even mention it - at least not for me.

Well, each to their own.

I still think that your criteria for DRM-free is short sighted, and a bit selfish. You don't mind DRM that affects other users but not you right now?

I usually don't know whether a game will run on my OS or not. I usually don't know whether a game will have compatibility issues with my hardware or not. I usually don't know whether it'll have issues with the hardware and OS I use next year or five or ten years from now.. I'm not too keen to end up like many gamers who cling on to some 10-year-old OS release in fear of compatibility breaking changes.

I also find that the vast majority of old games require some kind of tweaks to play smoothly. And where I used to tolerate old game jank pretty well, I've grown less tolerant over the years, which means community patches have become more and more important for me. Maybe the same will happen to you?

Unfortunately GOG has shown that they don't mind having restrictions that get in the way of patching things up. It's an issue on the radar. Yes, the number of affected games is very small right now but I think it's very important to keep an eye on this and stop it in its tracks before it becomes a widespread thing that everyone accepts because "it was always there." Of course, an extremely small number of gamers use GOG so maybe it's not worth mentioning GOG ;-)

What is DRM ? Digital Rights Management

In the context of a game, it is the management of the digital rights of the customer (player/gamer), by a party that is not the customer. Basically control of usage and ownership is not in the hands of the customer, such that the customer is pretty much just a renter who can have their rental agreement rescinded any time that other party feels like doing so.

That is the short and curly of it.

Further to that.

That managing party determine where how and if you can play the game ... under what conditions etc. It is an automated thing in most cases, so the customer has no-one immediate to engage with if there is an issue ... and maybe even no-one beyond that.

Of course it is selfish, I was ONLY talking abou myself. I can't and won't fight for everything and everyone, no matter if it's important to me or not. Nobody does this. If I had the choice of course I would chose the thing that would be the best for EVERYBODY but this is only about the question if I would be upset about this possibility missing.

Yes I(!) do. That does not mean that I would not prefer it the other way because of course I'd prefer everybody to be happy.

Well, you won't get a guarantee for a game running perfectly on your system even WITH the possibility of modding. Changing an OS will always lead to compatibility issues (at least in the case of MS) and this does not mean that somebody will try or be able to fix those issues. So again - no guarantee. The question for me is, how likely the cases you mentioned are. It's really rare for me that I have any issues with a game that is not broken itself.

I doubt it. I play PC games since they exist and now I am 50 years old and so far I never felt as if I would love to have all my old games modded/upgraded ... and also, again how often does it happen that a game prevents itself to be modded because of some DRM? 1 out of 100 games? I really don't understand what's the point in arguing here if the DRM'd cases are nearly non existant anyway - especially here on GOG.

So far I only know about FEAR and in this case they at least TRIED to fix the issue afair. Do you have a few more examples?