Even great businesses have short-term stock sell-offs.The S&P 500 is one of the most referenced measures in investing. This index includes 500 of the world's largest publicly traded companies and is often used as a proxy for how the overall stock market is performing. The index isn't just 500 random companies, but rather ones that are large enough and have met certain key criteria such as sustained profitability.While companies in the S&P 500 are generally strong businesses, even the best can go through rou ...