there are really two definitions.
1. Catch-phrase from Douglas Adam's book "The Hitchhiker's Guide to the Galaxy." Deceptively simple to understand, and yet this simple advice really can improve the quality of your life if you choose to follow it.
2. First song on Coldplay's debut album, "Parachutes," and also the first song on the "Garden State" Soundtrack. A short but very nice song. When you hear the song, and then read the book it becomes that much better.
"In many of the more relaxed civilizations on the Outer Eastern Rim of the Galaxy, the 'Hitchiker's Guide' has already supplanted the great 'Encyclopedia Galactica' as the standard repository of all knowledge and wisdom, for though it has many omissions and contains much that is apocryphal, or at least wildly inaccurate, it scores over the older, more pedestrian work in two important respects. First, it is slightly cheaper; and second, it has the words 'DON'T PANIC' inscribed in large friendly letters on its cover."
"Bones sinking like stones,
All that we fought for,
Homes, places we've grown,
All of us are done for
we live in a beautiful world,
Yeah we do, yeah we do,
We live in a beautiful world"