The British one pound (£1) coin is a circulating denomination of the Pound Sterling. The coin was introduced in 1983 to replace the Bank of England one pound note which ceased to be issued at the end of 1984. The original coin was round and was made of nickel-brass. A large number of reverse designs were issued in this format until it was discontinued in 2016, and its legal tender status withdrawn at midnight on Sunday 15th October 2017.
The new 12-sided £1 coin was introduced into circulation in the United Kingdom on 28th March 2017, with some of the coins minted earlier and dated 2016.
The coin has a 12-edged shape, similar to the pre-decimal brass threepence coin; it has roughly the same size as the previous £1 coin, and is bi-metallic like most £2 coins. Bimetallic means it is made of two metals - the outer ring is gold coloured (nickel-brass) and the inner ring is silver coloured (nickel-plated non-ferrous alloy).
A security feature is a "latent image" under the Queen's portrait - something like a hologram that changes from a "£" symbol to the number "1" when the coin is seen from different angles. Another security feature is micro-lettering - very small lettering on the lower inside rim on both sides of the coin, "ONE POUND" on the obverse (“heads”) side and the year of production on the reverse side, for example 2016, 2017 etc.
The new design is intended to make counterfeiting (which affected the earlier type significantly) more difficult, and also has an undisclosed hidden security feature called "iSIS" (Integrated Secure Identification Systems), thought to be a code embedded in the top layer of metal on the obverse of the coin, visible only under a specific wavelength of ultraviolet light.