A broker's demand for more funds when a leveraged account falls below the required maintenance level; positions are liquidated if you do not meet it.
Margin and leverage. A $5,000 deposit can control a $100,000 position, which is 20:1 leverage. Because the loss is measured on the full $100,000, a 2.5% move against you halves the deposit and brings a margin call, and a 5% move uses all of it.
When losses reduce your equity below the maintenance-margin, the broker requires you to deposit cash or close positions. Many brokers now liquidate automatically without waiting for you.
Margin calls happen at the worst time, forcing you to sell into weakness. This is why leveraged positions need stops far tighter than the margin math implies.
Example: you buy $20,000 of stock with $10,000 cash. If the maintenance requirement is 25% and the stock falls to $13,000, your equity is $3,000 (23%), below the line, and the broker calls for about $250 or sells stock.