An instrument that is secured by an ownership stake (equity), debt obligation (note, bond, etc.), or contractual right (like options and other types of derivatives) to buy or sell an asset (a commodity, financial instrument, etc.)

Originally, a security was a physical evidence of ownership: a paper document or certificate that represented an investment in an instrument. Over time, technology introduced other means as evidence of ownership: electronic bookkeeping and documentation.

The term “security” is now used to denote the instrument itself, rather than the evidence of ownership.