Seriously; what did 18,000' do to earn the title of FL180 (Flight Level One Eight Zero), when 17,000' is just 17,000 (One-Seven/Seventeen Thousand)? Here's some food for thought that may enlighten inquisitive minds:
The Altimeter
You already know that as you fly, you traverse regions of varying barometric pressure. Though your altimeter may indicate you're cruising at 8,000 feet above mean sea level (MSL), that indication is based on your altimeter setting (usually received by the last ATC controller with whom you spoke or by the ATIS at or near the airfield from which you departed). As you fly, you are (hopefully) paying attention to updates to the local altimeter setting by hearing it from ATC or by monitoring local ATIS reports. With each update, you will climb/descend to get back to (almost) exactly 8,000' MSL.
29.92
You may have heard about the magical 29.92 altimeter setting (called "two niner niner two," without the decimal). If you flew at 8,000' MSL from point A to point B far away, but left your altimeter untouched/un-updated, you would not actually be flying at the same exact altitude MSL throughout the flight. Instead, your aircraft would climb/descend to whatever altitude gave you an altimeter reading of 8,000'. For example, if you flew into an area of higher pressure, your altimeter would "feel" more pressure on it and indicate a lower altitude, so you would (naturally) climb until the altimeter reads 8,000' again. Inversely, flying into lower pressure would trick your altimeter into showing a higher altitude, encouraging you to descend until 8,000' is again indicated. That's dangerous!
Transition Altitude
This is the altitude at and above which you are no longer supposed to update your altimeter; instead you set it to 29.92 and leave it there. In the United States, the Transition Altitude is usually 18,000' MSL. Why? Ok, I really don't know. Here is my educated guess, though: Given what happens if you don't update your altimeter regularly, your aircraft my drop dangerously lower than 18,000' MSL by simply flying into an area of low pressure. That could potentially put you in danger of terrain obstructions (those pesky things called mountains). I don't think there are many mountains higher than that in the continental US (geography experts, chime in here). So, if you're flying up to and including 17,999' MSL, keep your altimeter updated with the local altimeter setting. If you're flying at or above 18,000' (on an IFR flight plan, since that is Class-A airspace in the United States), set it to 29.92 and let your altitude increase/decrease with the outside pressure changes.
Butt
But... What if I'm flying VFR at 17,500' and you're flying IFR at 18,000' and we're in a low pressure area? While I have my altimeter set correctly so I know I am flying at 17,500, your altimeter is set to 29.92 and you're actually flying lower than 18,000'! Don't worry; ATC has you covered. The minimum assignable flight level actually changes to prevent this from happening! According to the FAA's Joint Order 7110.65v (the "ATC Bible" / not to be confused with how you might spend money in Washington or Colorado), the lowest usable flight level can be increased to as high as FL200 if the pressure gets low enough (altimeter settings lower than 28.91).
...What?!?
The Cliffs Notes terms: Flight Levels are altitudes at or above 18,000' at which you no longer update your altimeter; instead, you set it to 29.92 and leave it there until you descend below 18,000'. :eagerness:
Ten Thirteen... that's what the rest of the world calls 2992. 1013 millibars (mb) or hectopascals (hPa). (Actually 1013.25, but who's that fussy?) If you're flying outside the US, expect ATC to talk to you in millibars not inches.
Also... if you're interested in simulating ATC outside the US realistically, there are two other terms you need to know: QNH and QFE. QNH is what you set your altimeter to to read your altitude, i.e. how far above sea level you are. Naturally that depends on the current local weather. QFE is what you set your altimeter to to read your height above the runway threshold. Which runway? The one where your ATC is based! If you're doing touch and goes in the circuit, ATC will always give you QFE - which is convenient for getting nice precise circuit heights and it will read zero as you touch down. If you're arriving cross country from a distant airport, you will also be given QFE because it's assumed you'll be landing there and will want to harmonize with the circuit. However, if you're departing, ATC will give you QNH, as you need something that's not peculiar to your home airport. (There's also Regional QNH, that ATC will give you on request, which is the aggregate low of all the QNHs in that ATC's particular region.)
:emmersed:
Now I know the differences between my QNH and my QFE!