[Alex3917]

"Where can one look up the actual legal definition for any given word?"

Words don't have universal legal definitions. However, at the start of a law there is usually a list of terms and how they are defined for the purposes of that law. So if want to know whether or not you can be prosecuted for X, you need to look up how the relevant terms are defined in the laws for X.

In the case of emails and phone records, in many cases the NSA isn't intercepting that information. Rather, the ISPs/telcos are intercepting it and sending the data to the NSA.

[rhizome]

And many times the terms are defined as a reference to another part of the law. I'm not sure if there is ever a chain of references ("...as defined in secA, secA says: see secB, etc..."), but I wouldn't be surprised.