"Insider selling" refers to the action of company insiders, such as executives or major shareholders, selling their own shares in the company's stock. It is a term used to describe when individuals who have access to private information about the company's future prospects, financial performance, or other important news decide to sell their ownership stakes. This can sometimes be seen as a signal that these insiders may have a negative outlook on the company's prospects, so it is often watched by investors to gauge the confidence of those inside the company.
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