The amount a client is required to have in his account in order to open/ keep his positions opened. Since a trader is allowed to use more capital than the amount, he/she deposited (due to leverage) a margin is required by the broker to ensure that any losses are covered. 

We calculate the margin with following formula: LOTS X CONTRACT SIZE/LEVERAGE Examples for Standard Account: If a client trades 1 lot of EURUSD in a Standard account with leverage 1:500, his required margin will be: 1 * 100.000 / 500 = 200 EUR Examples for Cent Account: If a client trades 1 lot of EURUSD in a Cent account with leverage 1:500, his required margin will be: 1 * 1000 / 500 = 2 EUR.