Definition
A margin call is a demand or account alert indicating that additional eligible equity or reduced exposure is required to satisfy margin rules.
In market context
A call can result from losses, increased requirements, concentration, or a decline in collateral value. The firm may set a short response window or take protective action immediately, and it is generally not required to preserve the customer’s preferred holdings when selling positions. A displayed warning should therefore be treated as an account-health event, not as a guaranteed grace period before liquidation.
Risk context
A firm can liquidate positions before funds arrive or without contacting the customer when its rules and market conditions permit.
Source
Use the primary source for fuller regulatory or market context.