If you're defining some technical/abstruse words at the beginning or end of the book, then to my mind it's a glossary. For a glossary -- as in a dictionary or lexicon come to that -- you wouldn't need to define eg every part of a verb, so if "bob" is a verb, you don't need to include bobed or bobing. But a glossary need only deal with words which need to be defined or explained, ie ones you've made up.
A dictionary would usually include all words, even familiar ones, but a lexicon might only include words relating to a particular topic or subject -- so if I were compiling a lexicon of architectural terms, it would only deal with words associated with architecture.
That help?