What Is the Term for the Relationship between an Option’s Delta and Its Implied Volatility?

The relationship between an option's Delta and its implied volatility is measured by a second-order Greek known as Vanna. Vanna measures the rate of change of Delta with respect to a change in implied volatility.

It is also equivalent to the rate of change of Vega with respect to a change in the underlying asset's price. Vanna is an important Greek for traders managing both directional and volatility risk.