A Pitched Battle is a major conflict that usually involves lost of men on all sides. It comes from the sixteenth century where battlefield communication was somewhat suspect. It would not be unusual to have armies wandering around the countryside for days trying to find each other. Realizing this was a huge waste of killing time and resources important battles were formally arranged ahead of time. Basically “we’ll meet you here at this time”.
So the Old English word picung which would become pitch means “to throw something into the ground (like a spear). That evolved into one of the modern day definitions of pitch, set up and fix in a definite position. Thus a pitched battle is one planned for a fixed and definite position.