A bear market is a sustained decline in asset prices, conventionally defined as a drop of 20% or more from a recent peak.
The label is a description, not a prediction — it says where prices have been, not where they are going. Historically these periods have been shorter than the recoveries that followed them, but nobody knows the length of any particular one while it is happening. The practical risk is rarely the fall itself; it is being forced to sell during one, which is exactly what an emergency fund exists to prevent.