A docket number is the number assigned by a court to a lawsuit.
U.S. Patent and Trademark Office Edit
A docket number is a number assigned by an attorney (or customer) to help them identify the files and related papers for a particular patent or trademark application. A docket number is simpler to use than the entire title of a case and less confusing — an attorney's clients could have multiple proceedings or case files with very similar labels. It is also easier to search for a short number than for a long title of a case file. There is a greater chance of an exact match on a simple number than there is for an exact match on a title when searching for it.