At the money
The state in which the price of the underlying and the strike are practically the same.
How to read the number
Time value is at its largest here, and the sensitivity of the premium to price changes faster than in any other state.
When the metric lies
A position at the strike before expiry is the least certain of all: the outcome is decided in the closing minutes, while the exchange asks for collateral as if it were already a full futures position.
Also known as: ATM option