For security (collateral), the legal right given to a creditor by a borrower, see security interest

A security is a fungible, negotiable instrument representing financial value. Securities are broadly categorized into debt securities, such as banknotes, bonds and debentures, and equity securities, e.g. common stocks. The company or other entity issuing the security is called the issuer. What specifically qualifies as a security is dependent on the regulatory structure in a country. For example private investment pools may have some features of securities, but they may not be registered or regulated as such if they meet various restrictions.

Securities may be represented by a certificate or, more typically, by an electronic book entry. Certificates may be bearer, meaning they entitle the holder to rights under the security merely by holding the security, or registered, meaning they entitle the holder to rights only if he or she appears on a security register maintained by the issuer or an intermediary. They include shares of corporate stock or mutual funds, bonds issued by corporations or governmental agencies, stock options or other options, limited partnership units, and various other formal investment instruments that are negotiable and fungible.