An object that is registered with a central
website (e.g. geocaching.com) and is then placed into a
geocache, often with specific
instructions or
wishes about where the object wants to
travel.
When other geocachers come upon a cache contaning a Travel Bug, if they are entertained by the idea, can take the travel bug out, and report that the bug has been found. Depending on the wishes for the bug's travel, that person will (hopefully) later move that travel bug to another cache that suits the bug's travel desires or else helps it meet is destination.
Travel Bugs can have high intentions, such as traveling from Seattle to Europe, or simpler aims in life such as visiting as many caches as possible, or moving around randomly as an objective in a geocaching game, such as Geo Hot Potato.
A travel bug can be anything, but is usually a representation of some animate object, such as a small plush doll, or little ceramic cat, or toy bug.