Marketable Security

You can sell it. If a security is marketable, there are (at least notionally) buyers for it.

Public securities with no 144a restriction or IPO lock up or other restriction...because you, the owner, are an insider with inside information...are marketable. Or even if those restrictions apply and then your company lawyer confirms that you are clear to trade, then those securities are marketable. You can ask your broker to market them, or "ask" for a price, and voila!...they get sold.

Private securities are a whole other matter. Many are sellable notionally, but the company provides no information about the financial performance (so what outsider would then have any idea what to pay for them?). Or the private shares carry ROFRs, or rights of first refusal, for the company to buy them back before any outside party can buy them. They're still then notionally marketable, but like the Facebook relationship status, you'd tag those securities: "It's Complicated."