A pokemon encountered in the wild with an alternative coloring; they also sparkle as they come out of your pokeball once you catch it - which is highly advised, seeing as there is a 1 in 8,192 chance of encountering one each battle. It's a pretty good idea to use a master ball on one if you see one, as the pokemon can sometimes flee before you get a chance to throw your pokeball or you can accidentally weaken it too much and KO it - which is a little like liquid gold running through your fingers.
Josh encountered a shiny pokemon - a Bellsprout - last week; he was so excited that he jumped up and down and accidentally made his Charizard use Fire Blast. He is now in the midst of a deep depression.
An alternatly colored pokemon,(e.g. Shiny Wingull)that shines upon coming out of it's ball.
Shiny Pokemon are twice as powreful as thier normal counterparts.