A simple definition of a security is any proof of ownership or debt that has been assigned a value and may be sold. (Today, evidence of ownership is likely to be a computer file, while once it was a written piece of paper.) For the holder, a security represents an investment as an owner, creditor or rights to ownership on which the person hopes to gain profit. Examples are stocks, bonds and options.
The Securities and Exchange Act of 1934 provides this more complicated definition, but you might want to grab a cup of coffee: "The term 'security' means any note, stock, treasury stock, 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."