Earlier editions frequently make references to the "adventure" as if it were a discrete unit of some sort. I've never known exactly what it is. Is it, like, one game module? One game session? One game day?
Here's an interesting passage from the 2e DMG, in which "adventure" seems to be used as a synonym for "game session":
Most passing time occurs within a single adventure: Spells rarely carry over from adventure to adventure (unless the session is stopped with the characters lost in winding caverns or the like); rounds of combat, while taking several game minutes, don't affect or spill over into subsequent adventures; days of travel often have no effect other than healing and the consumption of supplies.
If the DM wants, this is the only sort of timekeeping required. Time passed in previous adventures has little or no effect on the current session--each session or adventure is distinct and separate. For example, in one adventure, the characters spend a few hours in the dungeon, get injured, have some success, and return wounded. The night's game session ends with them returning to their home base. Next game session, the DM announces, "A week or so has passed since you last went out. Everybody is healed and rested. People with spells can pick new ones." The DM has chosen not to worry about the passage of time in this instance. An entire campaign can be played this way. Here's another example: In one adventure, a group of characters travels for three weeks and has several encounters, ending camped outside some ruins. The next session starts after the characters have camped for five days, so they can heal their wounds. Several hours pass as they explore the ruins, but no one is particularly hurt when they return to camp, and the game session ends.
The next session starts the morning after their previous adventure, everyone having gotten a good rest. The characters set out again. They spend a week on the road and arrive at a village. Here, the mage insists everyone wait while he researches a vital spell. Again, the game session ends. The next session begins two months later, after the mage has learned his spell and continues from there.
It's difficult to read "adventure" as anything other than a synonym for "session" in here. If it did, a "session stopped with the characters lost in winding caverns" would not, as it does, constitute two adventures. Also, constructions like "In one adventure... Next game session..." suggest that there is no distinction in the authors' minds between the concepts.
And what do you make of this advice from the D&D Companion set?
After reaching "Name" level, characters should gain a new level for each 3 to 8 adventures. More adventures can cause player frustration; fewer adventures can make the game too easy, and eventually bore them. If you play twice or more each week, 6 to 8 adventures per level gained is recommended. If your games are once a week or less often, 3 to 5 adventures per level are recommended.
Again, "adventure" here may well be a synonym for "game session". If not, what's the relevance of the number of times you play per week? If it doesn't mean game session, what does it mean? One castle? One dungeon? One module?
On the other hand, "adventure" is sometimes used as a synonym for "module". The D&D Expert set says, for instance, that "The Isle of Dread is a wilderness adventure designed for use with the D&D Expert rules." There's no way that the Isle of Dread is meant to be played in one session. And there's no way that characters are meant to play through 3 to 8 giant modules like Isle of Dread in order to gain one level.
Is "adventure" one of those words like "level" with a lot of meanings?
I have a feeling that "adventure" usually means something vague: not exactly "game session", but maybe "the amount of exploration that the DM thinks will take approximately one game session". But I'm interested in what you guys think. Is there some old-school assumption about this word's meaning that never made it into a rule book?
Lots of games have explicit game-session mechanics. Savage Worlds give you so many bennies per game session, for instance. Many D&D fans reject the idea of something that artificial as an explicit rules element. However, it seems to me that the game session has been baked into D&D since at least the 80s.
The funny thing is that 5e developers say that they're thinking of returning to the "adventure" as an explicit challenge-balancing mechanism. If you take them literally, that sounds like a return to the game session as as a game mechanic.