That's true, but what I think they generally refer to is the price the market would pay (that is, the price/share where demand for a share will equal the supply

That's true, but what I think they generally refer to is the price the market would pay (that is, the price/share where demand for a share will equal the supply). You are correct that "X is not worth anything unless and until X is sold" - that is why they are saying that IPOs may be lower than expected for these huge valuations - because the equilibrium market price is believed to be lower than the current valuations, priced by a small market of investors. According to the efficient market hypothesis stocks on the stock market will trade at their fair value (that's pretty disputed, but a good amount of respected people agree with it).