"Retardation" is not a standard term in physics. It's a colloquial term often used to describe deceleration, which is a decrease in velocity.

Deceleration is measured in the same units as acceleration, which are typically:

* Meters per second squared (m/s²): This is the standard unit in the International System of Units (SI).

* Feet per second squared (ft/s²): This is often used in the United States.

Important Note: While "retardation" is sometimes used, it's generally discouraged in scientific contexts because it can be considered offensive. Always use the term "deceleration" instead.