Market Value
  
See: Market Capitalization. See: Equity Capitalization.
The market value of pretty much anythiing...is whatever someone is willing to pay for it now-ish.
The stock market runs along, with prices changing by the moment. Comfortingly, the total shares outstanding for most companies change at a much slower pace. So if whatever.com has 50 million shares outstanding and its stock is trading for 40 bucks a share, then the Street is saying that the Market value of that company is $40 x 50 million, or $2 billion.
Think about it from the perspective of a home owner who spent $1 million building her amazing treehouse home. The best offer from anyone willing to buy it? 300 grand. That's the market value of the home, despite what Zillow says it should sell for.