Bear market
A sustained fall in market prices, commonly dated from a peak once prices have dropped 20 percent or more. A fall of around 10 percent is usually called a correction instead.
A bear market is the downward counterpart of a bull market, measured most often from the highest close before the decline began. Short of that, a fall of roughly 10 percent is normally described as a correction. Neither figure is set by a regulator: both are press and industry conventions that stuck because they are easy to check and easy to say.
Falls and recoveries are not symmetrical, and that asymmetry is where most of the confusion sits. A 20 percent fall needs a 25 percent gain to get back to where it started, because the gain is earned on the smaller balance that is left. A 50 percent fall needs 100 percent. In general, recovering a fall of takes a gain of , which climbs steeply as the fall deepens.
The mistake is treating a quoted fall as a realised loss. A falling price changes what the market would pay today; it is realised only on a sale, and whether it turns out to be permanent depends on what the asset does afterwards, which the label says nothing about. The opposite error is just as common, reading every 20 percent fall as certain to reverse. The convention names the size of a move that has already happened and nothing else.