An encounter is what happens when you spend an adventure. There are many different types of encounters, and to simplify things we'll discuss them in approximately the order in which they are checked by the game. (The exact order of some of these things isn't known completely. We know that 1-5 happen before the rest, but not how they relate to each other thanks to their rarity. The displayed order for those is somewhat arbitrary.) If after checking to see if you qualify for an encounter type you do in fact qualify, you will get it and the checking stops. If you don't, it moves to the next one. How we know this order will be explained as we go along.
Note: while the following applies to the majority of zones, there are few that are handled through entirely different mechanisms. Examples of these are the various mines, shoring, etc.
For the visually inclined, here is an encounter flowchart. That shows the order of checking simply and quickly.
Drunken stupor works by setting your adventure zone to drunken stupor. This effectively eliminates most things further down in the taxonomy. However, before the game decides which stupor you get, it still checks the rest of the taxonomy, but they just can't be met (since most are tied to a zone). But you can get encounters that aren't tied to a zone (such as wandering monsters). SSPD adventures would be either superlikelies or noncombats, depending on how they're coded they could be either (see the respective sections for discussion on those).
Such as the Cobb's Knob lab key and maybe some others. Pretty rare and exceptional.
These are the monsters that you get on regular intervals during special holidays/events, such as Feast of Boris, El Dia de Los Muertos Borrachos, the nemesis assassins, during the sand event, and monster under the effect of the badly romantic arrow. Not tied to any zone. Will not show up if your semirare counter is expired, however it is still possible to override the semirare with anything else in the chain (such as a clover adventure).
Those rare adventures caused by using astral mushrooms. Known to override superlikelies. However, since it was possible to use infinite-occurring superlikelies such as Between a Rock and Some Other Rocks or Showdown to get a large number of flashbacks by repeatedly declining them, and this caused massive server load, this may no longer be true. It's unknown how that exploit was fixed.
If you're due for this and you don't get anything above, you get this.
If you have a ten-leaf clover in your inventory, your zone has a clover adventure, and you've made it this far, you'll get the clover adventure.
These are the special adventures that occur on predetermined intervals based on a counter you can see if you eat fortune cookies. There are some claims that superlikelies can override semirares, but they do not. (As an aside if you want to confirm this for yourself, there are a couple ways. You need a character who's due to get the Mega Gem from Mr. Alarm, or with access to an instance of hobopolis with the boss ready. If you adventure in one of those zones while your cookie counter is up you will get the semirare. This is often done in Burnbarrel Blvd. to farm Jar of squeeze)
Contrary to popular belief, superlikelies are not any noncombat that is unaffected by +/- combat, or any noncombat with a condition on it. In fact, they're not really noncombats at all (unless you use the word "noncombat" to mean "anything that's not a combat", which is awkward). Superlikelies can be affected by +/- combat, Mr. Alarm is one that is affected. So what is a superlikely? It's another class of encounters, and what's special about them is that every superlikely in the zone is checked when you adventure there, before normal combats/noncombats are considered. If you meet all of the conditions for a superlikely adventure (and didn't get any encounters above), you will always get that adventure. If you meet all of the conditions for multiple superlikelies, a 1DN is rolled to decide which you get (these include BM adventures). Helpful definition, right?
This definition would be very helpful if we all had access to the innards of KoL. But we don't, and our main way of detecting superlikelies is to look for encounters that appear to be noncombats but that aren't affected by +/- combat, hence that belief. (Another way is that they can override combats, noncombats, and other superlikelies. This is only helpful if you know you're due for a certain encounter, which is rare. But if you've ever had the stone rose or a page adventure in the oasis override an ultrahydrated, you've seen that they're superlikelies.) At any rate, most superlikelies fit that empirical definition, but not all do. That's the takeaway here.
These might seem like wandering monsters but they're not. They don't override semirares, clover adventures, or superlikelies (wandering monsters will override the latter two). This was done because superlikelies such as as Between a Rock and Some Other Rocks or the clan dungeon boss superlikelies could be used to get all of your hipster/artist fights for free. This opens up some metagame where you can use the hipster/artist when you want superlikelies but not combats or noncombats, but that's beyond the scope of this document.
These are the familiar encounters we all know and love: the majority of encounters fit this definition. I've grouped them together because the game decides whether or not you get one or the other at the same time. I'll discuss how you choose which adventure you get in a zone separately. But as one last word, combats and noncombats can have conditions on them, so not everything with a condition is a superlikely (for instance, the Strung-Up Quartet is a normal noncombat with a conditional).
For how the game decides whether to give you a combat or noncombat, see Combat-Noncombat Selection.