Investopedia Definition: A bear market is when a market experiences prolonged price declines. It typically describes a condition in which securities prices fall 20% or more from recent highs amid widespread pessimism and negative investor sentiment.
Bear markets are often associated with declines in an overall market or index like the S&P 500, but individual securities or commodities can also be considered to be in a bear market if they experience a decline of 20% or more over a sustained period of time—typically two months or more. Bear markets also may accompany general economic downturns such as a recession. Bear markets may be contrasted with upward-trending bull markets.
The Motley Fool Definition: A bear market is typically defined as a 20% drop from recent highs. The most common usage of the term is to refer to the S&P 500‘s performance, which is generally considered a benchmark indicator of the entire stock market.
However, the term bear market can be used to refer to any stock index, or to an individual stock that has fallen 20% or more from recent highs.
Wikipedia Definition: A bear market is a general decline in the stock market over a period of time. It includes a transition from high investor optimism to widespread investor fear and pessimism. One generally accepted measure of a bear market is a price decline of 20% or more over at least a two-month period.
A smaller decline of 10 to 20% is considered a correction.