Securities are tradeable interests representing financial value. They are often represented by a certificate. They include shares of corporate stock or mutual funds, bondss issued by corporations or governmental agencies, stock options and other derivative securities, limited partnership units, and various other formal "investment instruments."
Securities are generally considered able to be traded on an exchange, and therefore registered with the SEC, though some may never trade on an exchange and some are exempt from registration.
Due to the difficulty of creating a general definition that covers all securities, the United States SEC attempts to define "securities" exhaustively (and not very prcisely) as: "any note, stock, treasury stock, security future, bond, debenture, certificate of interest or participation in any profit-sharing agreement or in any oil, gas, or other mineral royalty or lease, any collateral-trust certificate, preorganization certificate or subscription, transferable share, investment contract, voting-trust certificate, certificate of deposit for a security, any put, call, straddle, option, or privilege on any security, certificate of deposit, or group or index of securities (including any interest therein or based on the value thereof), or any put, call, straddle, option, or privilege entered into on a national securities exchange relating to foreign currency, or in general, any instrument commonly known as a "security"; or any certificate of interest or participation in, temporary or interim certificate for, receipt for, or warrant or right to subscribe to or purchase, any of the foregoing; but shall not include currency or any note, draft, bill of exchange, or banker's acceptance which has a maturity at the time of issuance of not exceeding nine months, exclusive of days of grace, or any renewal thereof the maturity of which is likewise limited." --Section 3a item 10 of the 1934 Act.
In the U.S., dealing in securities is heavily regulated by both the federal authorities (chiefly the S.E.C) and state authorities.