I guess is sort of depends on what you are looking for. The online Flash Help site is a place to start. Just hit F1 inside Flash to get to this page:
There's a lot of content up there, but here are a few of the helpful pages:
ActionScript Documents - Definition of every little piece of code there is:
Language Elements - Some definitions on keywords, operators, etc:
Compiler errors - When you get ActionScript errors, here's where you can look them up:
When it comes to animation, this page is a starting point for some of the definitions, terminology, and how-to information:
Hope this helps!