In D&D, I think it started out meaning "an undertaking or enterprise of a hazardous nature". It was something the players planned.
Later, it tended more to mean "an unusual, stirring experience, often of a romantic nature". It became identified with something planned by a scenario designer.
In the first usage, the bounds of "an adventure" might simply be characters departing for, and returning from, the dungeons or wilderness. The key point being pointed to was that experience points were scored (which depended on getting treasure back to base). There might be a rule that one could advance no more than a single level per "adventure" -- but that one might get in more than a single adventure per session of play!
Here's some advice on building scenarios from
RuneQuest (1978):
An adventure area, whether it be section of forest, cave, old ruin, river, etc., should provide the Player with the following opportunities.
1. Experience in the use of most of his skills.
2. The opportunity to obtain treasure to pay for further training.
3. The chance to die in the pursuit of the above.
4. An enjoyable time while doing all of the above.
The focus in the early days of FRP was on what might be termed "site-based adventures". The referee mapped an environment and stocked it with peril and plunder, and the players' adventure was exploration of and confrontation with what was there.
That tends to run directly into and intermix with the "character-based adventure", in which the key map is one of relationships among characters. Players discover that "terrain" of conflicting values and goals by analogy with investigating the mazes of dungeon passages.
An "event-based adventure" takes on more clearly the structure of a sequence of
events linked by a plot. It can end up branching into a fairly complicated map, but the presence of the "arrow of time" tends significantly to limit back-tracking and other re-use of elements.
The very best scenarios, I suspect, combine features of all those kinds.