alphabetical list of terms relevant to a certain field of study or action

A glossary is a list of words and what they mean.  They are usually found at the end of a book or report that uses hard words to read or special words.  Websites about complicated subjects also sometimes have glossaries.

For example, in a book about philosophy that has the word "epistemology", there might be a glossary at the end of the book with the definition of this word.