Often used in risk arbitrage. Requirement under Section 13-d of the Securities Act of 1934 that a form must be filed with the SEC 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. The purchaser of such stock must also file a 13-d with the stock exchange on which the shares are listed (if any) and 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: Williams Act.