Way, way back in the long, long ago, TSR began releasing products to support the core Dungeons & Dragons and Advanced Dungeons & Dragons rulebooks. These products were labeled "Adventure Modules" and they differed greatly in content, design, and reception. Some were cherished and adored. Others were hated and reviled. All were accepted as "adventures". I have long believed that the varied response to these products was not only due to personal preference, but also due to the actual design elements of these "adventures", some of which were not really adventures at all.
If you break down the content of these "Adventure Modules" you will actually see they are two types of products---adventures and supplements. For purposes of this article, an adventure is a series of outlined events that happen without character intervention, and in fact the outcome of these events can only change through the actions of the player characters. A supplement on the other hand is a location, with plot hooks, encounters, and places to explore and perhaps even things implied to be going on in the background, none of which requires the PCs to do anything.
Take a look at some of the classic adventures of the past: Night's Dark Terror, Curse of Xanathon, Assassin's Knot, Saltmarsh Series, Ravenloft, Red Arrow Black Shield.
Curse of Xanathon fits the criteria of an adventure. Badly. It's a horrible railroad that forces the PCs against a high priest who is unkillable the first time they encounter him. Only after they are forced to run from him (another railroad) do they learn how to kill him. Everything in it points to bad adventure design, and yet it is still classified as an adventure because the PCs are required to DO something to further the course of events.
A much better example of an adventure done right is Night's Dark Terror which has the PCs traipsing all over the Duchy of Karameikos trying to stop a magic-user who is uncovering the secrets of ancient magic. If the PCs do nothing, this mage and his organization of slavers could potentially become very powerful.
Ravenloft is another good example. The PCs are transported to the demiplane (which some might call a railroad) and must DO something in order to escape. Sure, they can bounce around in the village and forest, killing vampires and werewolves and such, but pretty soon Strahd is going to come looking for them which gives it a sense of urgency that is not present in a supplement.
Assassin's Knot and the Saltmarsh series involve investigative work, with consequences for inaction (or incompetence). In Saltmarsh, if the PCs do nothing, the coastal village will eventually be overrun by sahuagin.
Red Arrow, Black Shield involves the PCs traveling from country to country in an attempt to rally support for their army. Each country swayed commits troops to the final mass battle. Again, the PCs are required to DO something to complete the adventure.
Were these the greatest adventures ever written? Of course not. At the time, I don't think the folks at TSR really had a clear idea as to what made a good commercial adventure which is why you see dungeon crawls, investigations, wilderness treks, etc. all labeled as "adventure modules" and plot hook railroads accepted as standard fare. I think if they would have been classified better and a little more thought given to player motivations, some of them might have been better received.
Now let's talk about a few supplements: Tomb of Horrors, Keep on the Borderlands, Isle of Dread, Temple of Elemental Evil, Castle Amber.
Tomb of Horrors not an adventure? Keep on the Borderlands not an adventure? Isle of Dread not an adventure? Madness!!! We've all run those adventures countless times!
But hear me out. As I said above, an adventure is more than just a mapped location with some plot hooks and encounters. There needs to be some sense of urgency, some sense that the PCs affect a course of events through their action or inaction.
Take a good long and honest look at the Keep on the Borderlands. You get the keep, some encounters in the wilderness, and of course, the Caves of Chaos. These are places to adventure in. They are not adventures in and of themselves. There is no reason for the PCs to get involved in anything happening in the text or on the map and there are no consequences for action or inaction. The encounters are mostly static. The PCs can raid the Caves and return to the keep at will, then return to the Caves, and although there are some minor suggestions for what the monsters might logically do to adapt, there is no sense of urgency for the PCs to do anything.
Compare this to the Lankhmar: City of Adventure supplement (or my own Guidebook to the City of Dolmvay). Like the Keep on the Borderlands, each has mapped locations, plot hooks, and encounters that set the stage for memorable adventures. However, none of these products are adventures in and of themselves. The PCs are not required to DO anything. They can wander around, pick fights, explore locations, etc., however, if they decide to do nothing, there are no consequences for inaction.
Can you have adventures using the Keep on the Borderlands, Tomb of Horrors, etc.? Of course you can! In fact, they are designed that way. However, again, that does not make them adventures. Anything you say about adventuring in the Keep on the Borderlands you can just as easily say about Lankhmar or Dolmvay.
Tomb of Horrors is an extreme example of this. It's basically a high-powered dungeon without the surrounding maps and encounters to give it context (i.e. a Caves of Chaos with no Keep). If the PCs leave the Tomb, no big deal. Nothing happens. Isle of Dread is the same way. You have an excellent map with tons of interesting locations and encounters, but if the PCs decide to leave the island, well it'll still be there if they wish to return. Castle Amber is basically just a romp through a magical castle (and can be considered something of a railroad as well).
Temple of Elemental Evil is basically a crawl through a large dungeon although one could argue that the evil rising in the old temple gives it an implied sense of urgency that pushes the module towards the "adventure" category.
I'd like to point out my own Blood Moon Rising adventure for a clear example of what I classify as an adventure. It is set during a festival. There are a number of events that take place during the festival, some of which surround the opening of a demon gate. On the last night of the festival, a host of demons will be unleashed.
That is the outline of the events that occur if the PCs decide to stay in their rooms with the covers pulled over their heads. Now, the kicker is that the PCs can disrupt this whole chain of events by getting involved, locating the demon gate, and closing it which prevents the demons from being unleashed on that final night. If they act: they save the village. If they do nothing: demons.
So in closing, a series of events that only vary with PC intervention. That is a published adventure. A location with plot hooks and encounters. That is a supplement.