A piece of shiny plastic you wear on your beret or similar head dress, often used as an aiming point for the toilet trench; or, if you have an iota of pride, the single most important item of dress you will ever own.
Can you wear 2?
During the Battle of Alexandria in 1801, the 28th (North Gloucestershire) Regiment of Foot was surrounded and simultaneously engaged to front and rear. The command "Rear rank, 28th! Right about - Face!" was given. Perhaps it is worth taking a moment to realise that they were two ranks thick - a 'thin red line' even for an infantry fight - and they were under attack by cavalry. Normal procedure for fighting cavalry was to have at least three ranks, the first of which would brace their rifles against thr ground to stop the enemy from charging over and killing all their mates in the rear ranks, and praying that their comrades behind could kill the enemy before they themselves were killed. The 28th had just the one line in each direction, and if any enemy cavalry had hit then they would have all been wiped out.
They stood back-to-back fighting off the French for four hours, under attack from vastly superior forces including Napoleon's 'invincibles' regiment, but refused to yield one step (there was of course no chance for any man to take a step back from his position, because his comrades were behind him shooting at the enemy). Eventually the French assault - comprising at least five regiments of infantry and cavalry - gave up trying to dislodge the Glosters and retreated.
For their exemplary conduct on that day, the regiment was awarded the unique distinction of wearing badges both front and back of the head-dress.