If you use the second solution (ie, several URLs, one per page/definition), your users will be able to bookmark URLs for each specific page, use those to come back to youor site or link to it, or send them to other people, or whatever they want -- which is good.
If you only have one and only one page for your whole website, people cannot link to specific pages/definitions ; they have no way to come back to one specific word ; and that's bad :-(
Same for search engines, btw (Even if not that important for search results) : if they only see one page, they will not index your content well... and you probably want your site indexed.
So, in your case, I would probably go with several distinct URLs (even the corresponding pages are all generated by the same PHP script, of course).
And same thing for search results : you probably want to give people the ability to link to search results, don't you ?
About the "speed" thing : well, with Ajax, you'll send one request to the server. Without Ajax, you'll still send one request (for a bigger page, I admit), plus the ones for the images, CSS, JS, and all.
You should read a bit about Frontend optimization (see Yahoo's Exceptional Performance pages, for instance) ; it'll help quite a lot, about that ;-)