Form that must be filed with the Securities and Exchange Commission (SEC) to report when an investor has acquired shares of common stock in a public company totaling more than 5%. The form must be filed within 10 days of the acquisition and can often alter the direction of the stock depending on the investors. For example, if Warren Buffet acquires 5% of common stock in company ABC, which is publicly traded, other investors may buy up a lot of the shares thinking that it is a good investment.