In the context of trading, a margin call is a situation where a broker requests an investor to deposit more money into their margin account due to a decline in the account's value. This usually happens when the market moves against the investor's position, leading to a situation where the equity in the margin account falls below the maintenance margin requirement set by the broker. If the investor fails to meet the margin call, the broker may liquidate the investor's positions to cover the shortfall, which can lead to substantial financial losses for the investor.