The tercio (Spanish pronunciation: [ˈterθjo], Portuguese: terço [ˈteɾsu], "third") was a Renaissance era military formation made up of a mixed infantry formation of about 3,000 pikemen, swordsmen and arquebusiers or musketeers in a mutually supportive formation. It was also sometimes referred to as the Spanish Square or Tercio Español. It was widely adopted and dominated European battlefields in the sixteenth century and the first half of the seventeenth century.
The tercio was used during the Thirty Years' War, though tercio formations had begun to suffer some serious defeats to more linear formations. After the town of Grantville was transported to the past, the Americans fought their first battle against a tercio at the Battle of the Crapper and broke it with their long-range rifles, M-60 machine gun, and rockets. Despite being hit with modern weapons, the tercio continued to advance for a few minutes, before finally faltering and breaking.
The tercio was a wide and deep pike and shot formation, derived from the Swiss pike square. Due to their size and close formation, tercios were slow-moving, and could not bring most of their weapons to bear in any given direction, though a tercio could point some of its weapons in any direction.
At least some Spanish infantry units were also known as tercios.