Market cap implies public company because by definition private companies are not traded publicly on a market. It's the same way start ups have "valuations" and not "market caps".
> private companies are not traded publicly on a market.
Incorrect. They aren't traded on an exchange, but they are certainly traded in a market. A market is any situation where people buy and sell to one another. Venture capitalists certainly buy shares in private companies. Similarly, a private car dealership, private family restaurant, etc can (and often are) sold from the original owner to a new owner. Thus, a market exists.
I think the term you are looking for is exchange. The distinction between a market and an exchange is an exchange creates more explicit rules on trading in order to make the goods traded commoditized.
For example, instead of buying corn at my farmers market where the differences in corn might matter for both price and quality (local, organic, some farmer just happens to grow more delicious corn), when you buy agricultural commodities from Chicago Board of Trade (CBOT), each contract is equivalent. X many bushes of corn at Y price. By making rules on quality and standardizing the goods/contracts/etc, an exchange helps to bring about price discovery and ensure the goods are priced correctly.
But it isn't inherent that exchanges cause the true price to be found or that OTC[0] markets are inferior or less accurate. It depends on numerous factors for whether a share price is being valued correctly.
Funny that you mention corn specifically, because all contracts are not delivered the same even on-exchange! The deliverable grade of corn is "#2 Yellow at contract Price, #1 Yellow at a 1.5 cent/bushel premium #3 Yellow at a 1.5 cent/bushel discount" per http://www.cmegroup.com/trading/agricultural/grain-and-oilse... .
Along these lines, I heard there were complaints in the UK's LIFFE market because excess rainfall was damaging French wheat crops, but even the poor crops were within the exchange's loose contract specifications.
But I suppose this is a bit of a tangent. The takeaway is that everything is traded on a market somewhere, but there are different sizes and standards for markets. Private companies can have some number of institutional investors while still keeping reporting requirements low (IIRC Facebook ran into this limit before their IPO). Ergo, every company has a market cap, but companies with fewer investors/owners are harder to measure.
Incorrect. They aren't traded on an exchange, but they are certainly traded in a market. A market is any situation where people buy and sell to one another. Venture capitalists certainly buy shares in private companies. Similarly, a private car dealership, private family restaurant, etc can (and often are) sold from the original owner to a new owner. Thus, a market exists.
I think the term you are looking for is exchange. The distinction between a market and an exchange is an exchange creates more explicit rules on trading in order to make the goods traded commoditized.
For example, instead of buying corn at my farmers market where the differences in corn might matter for both price and quality (local, organic, some farmer just happens to grow more delicious corn), when you buy agricultural commodities from Chicago Board of Trade (CBOT), each contract is equivalent. X many bushes of corn at Y price. By making rules on quality and standardizing the goods/contracts/etc, an exchange helps to bring about price discovery and ensure the goods are priced correctly.
But it isn't inherent that exchanges cause the true price to be found or that OTC[0] markets are inferior or less accurate. It depends on numerous factors for whether a share price is being valued correctly.
[0] http://en.wikipedia.org/wiki/Over-the-counter_(finance)