The issuing authority uses less or no more precious metal in a coin but retains the coins face value. Doesn't happen much nowadays because no circulating British coins contain any precious metals. The last time it happened was when silver was removed completely from British silver coins in 1947 and replaced by an alloy of copper and nickel.

changing to a lower state (a less respected state)

Debasement is the practice of lowering the value of currency. It is particularly used in connection with commodity money such as gold or silver coins. A coin is said to be debased if the quantity of gold or silver is reduced.

The act of debasing or the state of being debased.

The act of demeaning or insulting oneself to control the behavior of another person. [8

being mixed with extraneous material; the product of adultering