Definition
Margin is eligible collateral or account value required to open and maintain leveraged exposure rather than the full economic value of the position.
In market context
Margin supports potential obligations arising from a position and is recalculated as prices, positions, and account equity change. It should not be confused with a fee or with the maximum possible loss, because the account remains exposed to the position’s notional movement. Initial, maintenance, and platform-specific risk requirements can differ, and reserved margin is generally unavailable for other uses elsewhere in the account.
Source
Use the primary source for fuller regulatory or market context.
