Major knowledge about a publicly-traded company that is not available to the public. Information may apply to its operations, such as its quarterly earnings, or to intangibles, such as an executive's imminent indictment. Nonpublic information almost always impacts a company's stock price, either positively or negatively, when it is made public. Prior to 2001, the use of nonpublic information in investment decisions qualified as illegal insider trading. Since then, even the possession of relevant nonpublic information may cause one to run afoul of SEC rules, even if an investor would have made a particular trade anyway. See also: Rule 10b5-1.
Information about a company that is unknown to the public. Insiders, including corporate officers and directors, are prohibited from buying or selling their firm's securities on the basis of nonpublic information.