Dictionary.net

• A body of soldiers drawn up in the form of a square with an empty space in the middle.

1914 - Nuttall's Standard dictionary of the English language
By Nuttall, P.Austin.

• In a body of foot, soldiers drawn up in the form of a square with an unoccupied space in the middle.

1874 - Etymological and pronouncing dictionary of the English language
By Stormonth, James, Phelp, P. H.