The term "13-D" refers to a specific type of filing with the U.S. Securities and Exchange Commission (SEC). It is required when an individual or group acquires more than 5% of a company's voting stock. This filing provides transparency about significant ownership changes, helping investors understand who controls a company. The Schedule 13D form includes details such as the purpose of the acquisition, the source of funds, and any plans for the company. It is often used by investors, including activist investors, who may seek to influence company management or strategy after acquiring a substantial stake.