There are a number of acronyms and slang used in the Geocaching world.  Here are some of the more common ones that you will come across.

• Muggles - These are people who do not know what geocaching is and will usually be looking at your funny while you look under bushes for the cache.  Case should be taken not to reveal cache locations to Muggles as they may mistakenly move the cache or throw it away.
• FTF - First Time Find - The person who has the honor of saying they were the first person to find a cache after it was hidden.
• DNF - Did Not Find - You searched for the cache but were unable to find it.
• TFTC - Thank you For The Cache - This acronym is usually used when signing the logbook of the cache, as space inside a cache is limited and logbooks are small.  This is your way of thanking the owner of the cache for taking the time to find a good hiding spot and maintain it.
• TNLN - Took Nothing Left Nothing - You did not "trade" at this cache.  It is proper cache etiquette to document in the cache logs if you traded items or did not.
• TNLNSL - An extension of the above indicating you Signed the Logbook.
• XNSL - A variation on the above.
• CITO - Cache In Trash Out - A geocaching campaign similar to Leave No Trace.
• YAPIDKA - Yet Another Park I Did not Know About.
• BYOP - Bring Your Own Pen - Many caches are not large enough to be able to include a pen for you to sign the logbook with.  These are usually noted with BYOP.
• Swag - Treasure and trinkets found inside a cache.
• TB - Travel Bug - a registered trackable item that moves from cache to cache.