Bear market
A fall of 20% or more in stock prices from a recent high. A smaller fall of 10% or more is usually called a correction.
Bear markets are a normal part of owning stocks, and nobody can reliably predict when one will start or end. A loss only becomes permanent if you sell.
Money you need within about 5 years is safer in insured savings or short-term bonds, so a bear market can't force you to sell stocks at low prices.
Example
A $20,000 stock fund that falls 25% loses $20,000 × 25% = $5,000, leaving $15,000.