A bear market is a general decline in the stock market over a period of time. It is a transition from high investor optimism to widespread investor fear and pessimism. According to The Vanguard Group, "While there’s no agreed-upon definition of a bear market, one generally accepted measure is a price decline of 20% or more over at least a two-month period."

Examples
A bear market followed the Wall Street Crash of 1929 and erased 89% (from 386 to 40) of the Dow Jones Industrial Average's market capitalization by July 1932, marking the start of the Great Depression. After regaining nearly 50% of its losses, a longer bear market from 1937 to 1942 occurred in which the market was again cut in half. Another long-term bear market occurred from about 1973 to 1982, encompassing the 1970s energy crisis and the high unemployment of the early 1980s. Yet another bear market occurred between March 2000 and October 2002. The most recent examples occurred between October 2007 and March 2009, as a result of the financial crisis of 2007–08.

A bear market should not be confused with a correction, which is a short-term trend that has a duration of less than two months. While corrections are often a great place for a value investor to find an entry point, bear markets rarely provide great entry points, as timing the bottom is very difficult to do. Fighting back can be extremely dangerous because it is quite difficult for an investor to make stellar gains during a bear market unless he or she is a short seller.