Are the definitions from a textbook glossary (consisting of common terms) protected by copyright laws?
For some definitions it is hard to really say it any other way then what is found in someones glossary. I do realize a dictionary would be protected (as a unique collection), but not sure how far that pertains to a glossary of terms at the back of a book.
A glossary certainly can have copyrightable aspects including, among other things, the definitions as well as the organization of the glossary.
That said, copyright does NOT protect works consisting entirely of information that is common property and containing no original authorship (e.g., standard calendars, height and weight charts, tape measures and rulers, and lists or tables taken from public documents or other common sources).
Without seeing the glossary, it’s impossible to say how much of it is copyrightable. Copyright analysis is a fact specific inquiry. You might consider contacting a copyright attorney if this issue is important to you.