Standard & Poor's 500 Index - S&P 500

The Standard & Poor's 500 is a U.S. stock market index that's based on 500 major companies in the U.S. If you want to know how the U.S. stock market is doing, check the S&P 500. It's "the market," more or less, even though the popular financial press quotes the Dow (which is just the top 30 largest American firms). The Dow was highly relevant when there were 500 public companies in the world, 100 years ago. Today, eh, not all that representative anymore of the broader market.