A common term used to describe a prolonged, down-trending market.
Markets move in phases, a bear market is a downtrending phase where the momentum favors lower prices and selling pressure is greater than buying pressure.
Bear markets can differ among themselves, as all markets have different volatility, a 20% move to the downside may be considered to be a bear market in equities whereas in crypto, due to its volatile nature, it could be a regular correction in an uptrend.
In crypto bear markets, one may see losses of more than 90%.