Let's talk first about what a bull market is. The standard definition is the 20 percent rule. A bear market is underway when the S&P falls 20 percent from its bull-market high, and a bull market begins once it rises 20 percent from its bear-market low.
First, who decided it was 20 percent and what is so magic about that level? The short answer: no one knows exactly, but veteran market watcher Louise Yamada, who for many years was technical analyst at Smith Barney, told me that the idea goes back to her predecessor, Alan Shaw, who was one of the founders of technical analysis. Louise told me that Shaw would toss around the terms "bear market" and "correction" but to help traders understand the magnitude of the event he was talking about, he told them that when he said "correction", they should think of a decline of around 10, and a "bear market" would be a drop of around 20 percent or more.
Since Shaw began his career in the late 1950s and was a fixture on Wall Street for decades, this likely would have been in the 1970s, so the tradition goes back a long way.
There's plenty of people who dismiss the 20 percent rule, including the folks at the Stock Trader's Almanac, who nonetheless agree that March 9, 2009, was important. "It marked the end of a long severe decline in stock prices, the end of the bear and the beginning of the end of the Great Recession, which the National Bureau of Economic Research (NBER) determined ended in June 2009," said Jeffrey Hirsch of the Stock Trader's Almanac.
OK, so there's nothing magic about 20 percent. But here's a more important point. There are plenty of other ways to measure a bull or bear market. Some argue that a better way is to look at rallies to see whether they surpass the old highs. Under this measurement, the bull market really began in March 2013, when it finally passed the pre-crisis high set in October 2007.