Couple of online ones that are a useful quick reference:
here and
here
In my opinion the best aviation dictionary you can possibly get is Dale Cranes, "Dictionary of Aeronautical Terms". Published by Aviation Supplies and Academics (ASA). ISBN: 1560272872. The current revision was published last month. Available from Amazon, + others I'm sure.
This dictionary should answer most questions you will ever have, and fill you so full of other amazing information, you may turn out like me!
I would be interested in any other recommendations anyone else has.
Edited for badly behaved urling!
[ 26 October 2001: Message edited by: SuperSpanner ]