A margin call is a demand from a broker to an investor to deposit additional money or securities into their account in order to meet minimum margin requirements. This typically occurs when the value of the investor's account falls below a certain threshold due to market fluctuations. Failure to meet a margin call can result in the broker liquidating the investor's assets to cover the shortfall. Margin calls are a common occurrence in margin trading, where investors borrow funds to increase their buying power in the market.