[Jd]

The textbook definition of a security is expecting to profit based on the efforts of a third party.

[jessriedel]

The definition in Kadin's comment that I reference is more exact.

[runeks]

With what money will you buy the security unless you, yourself, have exerted an effort? You’re profiting from the money you’ve earned through exerting an effort, followed by foregoing consumption, resulting in surplus capital with which you can purchase the security.