Good digital dictionary?

I need a good dictionary for checking whether or not a given string is an English word or not. Do I need to approach this problem a certain way? I'll be needing to scan a lot of words, so I'm prioritizing quick lookup time.

If it matters, I'm working in Java. Suggestions?

>>12 Would there even be a way to do that? Is there a way to call a system utility from a script? There must be, I've just never heard of it.