Two or more people edit documents at the same time over a web browser.
Mostly people speak of real-time collaborative editing (RTCE) (Wikipedia (en)):
A collaborative editor is a form of collaborative software application that allows several people to edit a computer file using different computers, a practice called collaborative editing. There are two types of collaborative editing: real-time and non-real-time. In real-time collaborative editing (RTCE), users can edit the same file simultaneously, whereas in Non-real-time collaborative editing, the users do not edit the same file at the same time (similar to revision control systems). Collaborative real-time editors generally permit both the above modes of editing in any given instance.