My nitpick is that "tradable" is non-central to the definition, even though most things that people think of as securities are both tradable in theory and have

My nitpick is that "tradable" is non-central to the definition, even though most things that people think of as securities are both tradable in theory and have active markets in them. A security without an active market in it? Still a security. A security which can't be traded even theoretically? Still a security. A reason to make this nitpick is that promoters sometimes use "Oh this is just a commercial relationship between business partners and we're locking you into it in all sorts of interesting ways; therefore, we're not offering a security" as an argument. The government takes a dim view of it. (This was a major issue in the fractional orange groves case.)