Twitter wants to own the subtweet. In a recent filing, the company has sought trademark rights to the word "subtweet." Used colloquially, the word refers to a tweet that refers to a specific person without using that person's twitter handle. The term has been in public use since at least 2013, and does not appear to have originated with employees of Twitter Inc, but is rarely used outside the context of Twitter as a platform.
The application was filed on October 30th, just a few weeks after Jack Dorsey was named permanent CEO of the company, and subsequently published for opposition on November 19th. ("Sub tweet" is also included as a subsidiary trademark.) If no one opposes the trademark, it's likely to be approved in a matter of months. Twitter will need to use the term in a commercial setting to maintain the trademark rights, but otherwise it will have exclusive rights to authorize commercial use of the term.
Reached for comment by The Verge, Twitter provided the following explanation for the trademark: "When you need to protect your namespace from people who would misuse it. #keepingit"
1:53pm ET: Updated to include Twitter comment.