Two definitions are constantly present in the ATAG guidelines: authors and authoring tools.
Authoring Tool Accessibility Guidelines (ATAG) 2.0 glossary section on W3C has a lot more terminology explained.
Authors are people who use the authoring tools to create or modify web content. This can include roles such as designers, programmers, publishers, and testers, among others.
Authoring tools are any applications (web-based or not) that can be used by authors to create or modify web content. A couple of examples of authoring tools are content management systems (CMS) like Drupal and WordPress, and WYSIWYG (What you see is what you get) editors.