Margin

The amount your broker requires you to keep against an open derivative position. It can be raised while the position is open.

Where this comes up

Lessons that use the term in context, which is usually more useful than the definition on its own.

What you are paying for — Intrinsic and time value, separated properly.
Vertical spreads — Capping both ends on purpose, and what it costs.
Expiry day mechanics — Settlement, exercise and what actually happens at the close.
Sizing a position properly — From capital and stop distance to quantity — the whole calculation.