gahhh! Silly me, I've been linking to a non-existent
node all this time! Well, here's my writeup
A dead language is a language which no longer undergoes the natural process of mutation. Lexicography and minor points of grammar might change (it's not productive to simply say 'degenerate'), but under artificial conditions. The most common example is Latin. Thanks to the Roman Catholic Church, the country of Finland, and programs like the nuntii latini (a news broadcast in Latin), words and phrases are constantly being added, definitions change. But these are conscious changes; someone in the process of writing realises he can't find a word for computer and invents one. I've met the pope's chief Latinist; he's from Wisconsin. He's quite brilliant, rather fluent in the language, and has an awe-inspiring grasp of stylistics.
But this is not a natural change, such as the contraction 'gimme' from 'give to me' in English. Nobody learns Latin as a first language/child language (kindersprache) either, a crucial point in learning a language; thus, to varying degrees, thought patterns are not natively Latin, but translated.
Just because it is still in use, does not mean the language is suddenly alive. Latin is just as dead as Akkadian and Middle Egyptian.