A battlefield is the area in which a battle is fought.
Open ground was the the ideal in-period battlefield was one which was broad, unobstructed, and had firm soil suitable for a cavalry charge. The Battle of Bouvines was fought on just such a battlefield. The field battle at Pennsic is an SCA example of an open ground battle.
Broken ground was least advantageous to the mounted warrior, but favoured warriors on foot, especially in loose order formation like a skirmish line. Broken ground could be marshy, rocky, heavily wooded, or very hilly and uneven. In the highlands of Scotland, it was frequently all of the above. The woods battle at Pennsic often involves broken ground-style terrain.