Margin
Margin is the money you put up as collateral to open and hold a leveraged position.
Initial margin is the collateral needed to open a leveraged trade (the position size divided by the leverage). Maintenance margin is the minimum that must remain once the trade is open; if losses push your equity below it, the position is liquidated.
Isolated margin limits the risk to the collateral you assigned to one position, while cross margin shares your whole account balance between positions, which can save a trade from liquidation and can also draw down everything else.