Other forms: insiders
An insider is someone, usually one of a small group, who has access to secret or private information. A political insider knows what really goes on behind the scenes in a campaign or a senator's office.
In a large company, insiders are privy to financial information or details about the way things work — information that isn't available to an ordinary employee or the general public. In the financial world, an insider knows things about the businesses whose stock is being traded, and "insider trading" is when they illicitly use this information for personal gain.