Today the annual Trooping the Colour ceremony is taking place, when the Queen inspects her troops. But what do the words mean? The verb 'to troop' developed from the noun in the 16th century and originally meant 'to gather', soon gaining the meaning 'to march in rank'. The 'colour' (or 'colours' - it's often used in the plural in military use) is the regimental flag or ensign. Originally, the flag would be carried up and down the ranks of troops as a symbol of the regiment.