Naw, they're not. A lot of people/places (Sadly including GOG) use them interchangeably, but it's important to notice when things are "expansion" caliber.
Expansion: Large, sweeping content changes, beyond the realm of what modders or hackers would typically be able to do without extreme time or effort.
DLC: "First party paid mod". Typically something modders could do and release. Even if it's a lot, if it could be done with existing mod tools (or basic hacking of the resource and data files for the game).
Microtransaction: "Could be done by a save game editor, 'trainer', or cheat code." Which is why they really only work in a DRMed environment, because otherwise you could just do so. We do have some examples of them on GOG (some of those jRPGs that give you items in your save file if you have the bits in there; I don't think any of them introduce new items, but only put them into your saves).
The thing is, expansions existed before people paid for additional content online. People got them on CDs and DVDs. There were also "standalone expansions", even. So, both amok and you are right, in a way. He is right in that nowadays everyone gets game addons and, therefore, all is "downloadable content". You are right in that "expansions" was a label for major changes, and some people reserves the word for paid game addons of very large caliber.
But yes, all are dowloadable content nowadays, and online distribution made selling small chunks of addons economically viable, thus blurring the line. Some DLC are what you could get via a special code in the game, even cosmetic changes. Others add campaigns or major features, others change the face of the game so that you seem to be playing something rather different. The discussion is all about semantics.
The point is, "expansion" is about the scope of the addon, while "DLC" is about the medium you get the addon through. More solid terms ought to be coined. For example, if you want to consider the scope, you can ditch the "DLC" term and replace it with one or two expressions that are more enlightening. LIke, something like minor addon, cosmetic addon, campaign addon, major addon... then, expansion... or something like that. Surely someone somewhere will have started using something useful. Or we might start here in GOG! :-)
By the way, both of you get upvotes for caring about clear communication concerning games, even if the discussion is somewhat byzantine ;-)
Yes, you are right. There was additional content before we had downloadable content. But even then it was called epansions genrrally, no matter the size of the content. For example the expansions of one of the early car games (the name eludes my right now, one of the problems with age....) where the expansion pack was basically an addition of three new cars (which all played the same, they just looked different). I am not sure what Mqstout here would argue that this was. This is before the time of modding.... But it was sold as an 'expansion pack'.
The main difference between now and today is the ease to sell the expansions / additions / plug-ins / what-ever-you-want-to-call-them via the interwez vs physical. Back then you needed to print and distribute the disks, which meant that it had to be cost-efficient to do so. You coud not really small cheap extensions / whatever, it was just not viable. So things tended to accumulate until the point when it was deemed that it was sensible to release them. Today, there are no such limitations, so it is possible for a game to sell a skirt for one of the side characters in a slightly different shade of pink for £0.5. Back then it was just not possible.
However, when you go by the defenition of what DLC actually means, removing all kinds of predjudice and bias and objectivly looking at the word, the word as said before is an abrivation that spells out DownLoadable Content. That is all it means, there are no other values affixed to it. All content for a game that's downloadable is a DLC, no matter the size of the content or what the content is. There are nothing withing this term that dictates this in any shape or form.
Does this make the term DLC very nebulous? Yes, it does. It is a very vague catch-all term. Do we need others? maybe? we already have cosmetic DLC, which its implications, or expansions which its - these may be enouhg? or perhaps it would be good to have something clearer? But the point being that at the end of it all, if an expansion for a game is sold online and you download it - then de facto it is a DLC. You might not like this, but that;s how it stand.
So going back to the very original question "Is it an expansion or a DLC though?", you can perhaps see how I find the question flawed. In my oppinion it would be better to say something like "Is it an expansion or collection of a cosmetics and small fixes though?"
anyway, that was my rant of the day.