Agent A person acting on the author's behalf to sell their book to publishers. They may also act for the author in other ways such as booking events. They usually work for a percentage of the sales.
A.R.C. Advanced reader copy. These are copies of the final, or almost final, version of a book which are given to readers in exchange for an honest review. They can be physical or ebooks and the reviews could be anything from a big feature in a prestigious newspaper to a few lines on Amazon or Goodreads. As the name suggests these are usually sent out prior to the official release date, so that reviews are available as soon as the book goes on sale.
Backlist Books produced by an author or publisher in previous years and still in print.
Bio Biography A short piece of writing about the author, generally composed by them and written in third person. Usually these will be adapted to suit the piece of work being published and/or the place it will appear.
Blurb Often found on the back of the book, these are short descriptions of the book (without spoilers!) to entice potential readers.
By line The author's name or pen name on a published article or story. It's often preceded by the word 'by.'
Contract. The agreement between author and publisher (or service provider). The terms of these vary hugely. Always read them carefully, even if they look to be just like others you've signed. If there's anything you don't understand then ask. If there's anything you don't agree with then it might be worth asking if it can be changed. NEVER sign a contract until you're sure you understand, and agree with, all terms.
Copyright. This is ownership of the work. If you wrote it you're the copyright holder – until you sell or give away your rights. In order for your work to be published you will have to asign the right to do that. The rights requested can be anything from single use (the right to reproduce the work once in one format) to all rights (the author completely gives up the story and can never use it again in any way, profit from it, doesn't even have the right to be identified as the author (although usually they will be).
There are numerous variations between these extremes. It's in the author's interest to give up as few rights as possible, because those retained can be used themselves, or possibly sold to someone else. I do not recommend anyone ever give up all rights to their work.
Cover Letter Covering Letter A letter (or email) sent with a submission to and editor or agent.
Deadline. This is the date by which work must be submitted. It may be the closing date for a competition, or the agreed date to supply a commissioned piece. Ideally deadlines should always be met. If for some reason it seems this might not be possible, contact whoever commissioned the piece as soon as possible and explain.
D.P.S Double page spread. A story or article displayed over two facing pages.
End notes. Anything in a book which comes after the end of the story, or main content if it's non fiction. Often these will be acknowledgements and/or a list of other publications by that author. You do not have to include end notes.
I can see this will be a ridiculously long post if I keep going, so I'll stop for now and think of a sensible way to continue – probably by dividing it into alphabetical sections.