From Traderpedia

Definition: A call from the brokerage to the customer requesting that the customer deposit additional funds into their account in order to return the balance to its required level.

The margin call (or maintenance call) arises because of an adverse price movement in one or more of the securities held in the account. A margin call requires the account owner to bring the account value up to the minimum acceptable amount (maintenance margin).

This is sometimes also referred to as a house call.