The percentage of your available margin currently in use, a fast proxy for how close you are to forced deleveraging.
Utilisation = used margin / total available margin. At 25% you have room for requirements to rise and for positions to move; at 85% you are one volatility spike from a problem.
It is not static on the broker's side either. Exchanges raise initial-margin during stress - futures margins routinely jump 30-50% after a shock - so a position sized at 70% utilisation in calm markets can be over the limit without you trading at all. That is the mechanism behind many "I was right but got closed out" stories.
Set an explicit ceiling, monitor it daily, and treat approaching it as a signal to reduce rather than to deposit. Utilisation should be checked alongside max-open-risk: they measure different things, and a book can be light on margin while heavy on correlated risk.
Original diagrams for the ideas on this page. Illustrative, not real market data.
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.
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