Schedule 13D

A form that must be filed with the SEC under Rule 13D. The form is required when a person or group acquires more than 5% of any class of a company's shares. This information must be disclosed within 10 days of the transaction. Rule 13D requires the owner to also disclose any other person who has voting power or the power to sell the security.

Investopedia Says:
Once the disclosure is made to the SEC, the company and the exchange(s) on which the company trades are notified of the new beneficial owner.

You can find most company's 13D filings in the SEC's EDGAR database.

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Form required under Section 13d of the Securities Act of 1934 within ten business days of acquiring direct or Beneficial Ownership of 5% or more of any class of equity securities in a Publicly Held corporation. In addition to filing with the Securities and Exchange Commission, the purchaser of such stock must also file the 13d with the stock exchange on which the shares are listed (if any) and with the company itself. Required information includes the way the shares were acquired, the purchaser's background, and future plans regarding the target company. The law is designed to protect against insidious Takeover attempts and to keep the investing public aware of information that could affect the price of their stock. See also Williams Act.