Some of the more commonly used terms

A.P.E. Cache	A particularly obscure type of cache. There’s one in the USA and one in Brazil.
Ammo Can	Ammunition boxes or ammo cans are containers originally designed for safe transport and storage of ammunition. Ammo cans are popular containers for regular or large geocaches and are regularly stolen.
Archive	Archiving permanently removes a geocache listing from search results. A geocache owner can archive their own listing. A geocache owner cannot unarchive it. As an alternative to archiving, the geocache can be temporarily disabled if maintenance is going to be performed or the container will be replaced in the near future.
Attributes	Icons featured on a details page for geocaches that describe specific characteristics of a geocache. There are several classes of attributes, such as whether or not you need special equipment, possible hazards along the way, or unique conditions one should be aware of. Attributes can show whether or not a cache is wheelchair accessible, dog friendly, requires a flashlight, and more. Attributes are also a tool to help you filter the types of geocaches you would like to search for when building a Pocket Query. It's the sort of thing you take notice of once it’s too late
Basic Member	Someone who’s not paid up and consequently can’t see most of what’s on the geocaching map
Bookmark List	A Geocaching Premium Membership feature that can be used to group geocache listings in whatever way you like. You may want a Bookmark List of caches you intend to find this weekend or perhaps an "all-time favourite" list to share with friends. You won’t, but you could.
BYOP	"Bring Your Own Pen/Pencil". An acronym often used by geocache owners to communicate to other geocachers that you will need to bring your writing utensil in order to sign the cache logbook. If you don’t you’ll have to sign the log with blood or mud. I’ve seen both used.
Challenge Cache	A type of geocache that requires seekers to find a particular amount of specific caches in order to be able to claim a find. Usually the requirements are nigh-on impossible.
CITO	"Cache In Trash Out" is an ongoing environmental initiative supported by the worldwide geocaching community or so it says at www.geocaching.com/cito. It usually consists of walking round a pristine park in a fruitless search for discarded crisp bags before going to the pub.
C.O.	Cache Owner. The one who hid the cache. They can delete your logs, so don’t piss them off.
Coordinates	A pair of numbers (latitude and longitude) that pinpoint an exact position, or waypoint, on the Earth. Latitudes are horizontal lines on the globe that run parallel to the Equator. Latitudes never intersect, and one degree of latitude equals approximately 69 miles (don’t say I never learn you nuffink). Longitudes (also called meridians) are vertical lines on the globe that converge at the North and South Poles. They are widest apart from each other at the Equator. The Prime Meridian runs through Greenwichat zero degrees longitude.
Creed, The	Also known as the "Geocachers' Creed". Designed to help orient new players to the ethos of the geocaching community and to guide experienced players in questionable situations, so that everyone can enjoy geocaching. No – I’d never heard of it either.
Datum	Datums are different calculations for determining longitude and latitude for a given location. A datum is chosen to give the best fit given the true shape of the Earth. Currently, Geocaching.com uses the WGS84 datum for all caches. It’s a shame that no one else does.
D/T	There’s a separate section on that
Dipping	Logging a trackable into a geocache, and immediately logging it back into your possession. This registers miles traveled on a trackable. You can also achieve the same goal with a "Visit" log.
Disable	What the reviewer does to your cache listing when it’s missing. This status is meant to be temporary and should be resolved within a reasonable amount of time (cough cough).
DNF	"Did Not Find". An acronym used by geocachers to state that they did not find a cache when they don’t have a spare in their pocket to use as a throwdown.
EarthCache	An EarthCache is a special place that people can visit to learn about a unique geoscience feature of our Earth. EarthCache pages include a set of educational notes along with cache coordinates. Visitors to EarthCaches can see how geological processes have shaped our planet, how we manage its resources and how scientists gather evidence to learn about the Earth. And then send the C.O. an email saying “blah blah blah sandstone”. No? – that’s how it’s been working for me (as a finder and a C.O.) for years.
Event Cache	An Event Cache is a gathering of geocachers. Your first one can be a bit scary, but they can be good fun.
Find Count	The number of geocaches a player has found. It’s a fun number and doesn’t really count for much… all the time mine is higher.
FTF	"First to Find". An acronym written by geocachers in physical cache logbooks or online when logging cache finds to denote being the first to find a new geocache. It is worth serious bragging rights
GC Code	A unique identifier associated with every geocache listing. The GC Code starts with the letters "GC" (dur!) and is followed by other alphanumeric characters, such as GCK25B.
Geocoin	Geocoins work similarly to Travel Bugs®. They are coins with a unique code that can be attached to an item. The geocoin is then carried from cache to cache (or person to person) in the real world, and its progress can be followed on Geocaching.com. After a few months the serious cacher has spent over a hundred quid on these and doesn’t dare release any.
GPS Maze	The GPS Adventures Maze is a traveling educational exhibit developed to teach people of all ages about navigation, GPS technology and geocaching. A hands-on experience that … blah blah blah… people just go because it’s another icon type.
GPX (GPS eXchange Format)	A common GPS data format used to describe waypoints, tracks, and routes that can be interchanged between GPS devices and software. Stick in GSAK and hope for the best
Ground Zero	The point where your GPS device shows that you have reached the geocache location. At ground zero, you are zero feet (or zero meters) away from your destination even though you can clearly see nothing.
Groundspeak	The company that owns and operates Geocaching.com. We have to be nice about them.
GSAK	A frankly wonderful bit of software which sticks all the info about the geocaches in a given area onto your GPS and save farting around with needing internet signal… and then uploads all your logs when you get home. Download it from here. Seriously. Do it now.
LN	"Left Nothing". A common term used when a geocacher leaves nothing in the geocache and simply signs the logbook. I like seeing that – there’s enough tat in my caches already.
Locationless Cache	A legacy cache type considered the opposite of a Traditional Cache. Instead of finding a hidden container, geocachers locate a specific object and log its coordinates. There’s usually one of these to be had every year or so if you pay attention.
Logbook	A physical record of everyone who has found a geocache. Ideal for comparing to the electronic record if you are looking for a petty argument with someone you’ve never met.
Pocket Query (PQ)	A Geocaching Premium Membership feature popular with people who like doing things the difficult way. Go on – get GSAK now.
Premium Member	Geocaching.com members with a paid Geocaching Premium Membership. Premium members have more features available to them than Basic members but still need GSAK and Project GC to do the hobby properly.
Reviewer	Community volunteers from all over the world who review geocaching listings for content and publish cache listings on Geocaching.com. They don’t get paid, they do get a load of stick.
ROT13	A simple letter substitution cipher, or code, where each of the letters are rotated 13 characters up or down in the alphabet. Hints for geocaches are encrypted using ROT13 Decryption Key A|B|C|D|E|F|G|H|I|J|K|L|M ------------------------- N|O|P|Q|R|S|T|U|V|W|X|Y|Z (letter above equals below, and vice versa)
Signal	Signal the Frog is the official mascot of Geocaching.com and is often seen walking around Mega events being harangued to pose for a photo.
SL	Written on on-line logs to say that you "Signed Log". Well, of course you did. That’s the rules.
Spoiler	A spoiler is information that can give details away and ruin the experience of the find. Leaving them is a good way of getting your log deleted.
STF	"Second To Find". The second person to find a geocache after it has been placed. Who cares?
TFTC	"Thanks For The Cache". An acronym written by geocachers in logbooks or online when logging cache finds. It’s looked down on…
TOTT	"Tools of the Trade". An acronym used for any of the tools that might be used to search for/retrieve/find/log a geocache. Usually a small stick.