Motorbikes are fairly diverse : classic, speed, tuning, stunt, touring, custom and so on.
    When you have a closer look, there are even more subcategories. For instance all-terrain bikes=trial, cross, enduro, rallye-raid, adventure...
    I'll give a try to define adventure bike from its potential use and the features which result from.
    Very few manufacturers (KTM, BMW) are selling ready-to-start adventure bikes
    A bigger pack can sell other machines which, after some  good preparing work, can run on similar paths. Some examples : Suzuki has the 400 DRZ and V-Strom, Aprilia the Pegaso Trail and Caponord (no need for a compass !), Honda the 1000 Varadero, and even Buell the XB12 R. I won't try to be exhaustive.
    The Triump Tiger unfortunately has lost the major part of its adventure skills.  But If you like it in its previous version have a look at these sites :
http://www.twotaildog.com/Motorcycling_the_Great_Divide/00Intro/Great_Divide_Intro.htm to see what you can do with it,
and for the "prep" http://www.twotaildog.com/Motorcycling_the_Great_Divide/Great_Divide_the_Tiger/Great_Divide_Tiger.htm
    An Adventure Bike must be simple and strong, comfortable, with a big tank, good all-terrain skills, and be able to carry a lot of luggage, among other requirements.
    Actually, each bike needs to be further prepared, except maybe for limited series : example : the KTM 640 witch comes with good alu cases :

    A true nearly-ready-to-adventure one, but there still are accessories to put on:
...
    You're not really obliged to make those big trips to enjoy it. Let's say Scotland for British or Alps for other Europeans can be a fair trip too. But it's easier to fancy about crossing Eurasia, Africa, or America with such a bike.
    The best source for Adventure riding on the net : http://www.horizonsunlimited.com/