A pitched battle or set piece battle is a battle in which both sides choose the fighting location and time. Either side has the option to disengage before the battle starts or shortly thereafter.
A pitched battle is not a chance encounter such as a skirmish, or where one side is forced to fight at a time not of their choosing such as happens in a siege or an ambush. For example, the first pitched battle of the English Civil War, the Battle of Edgehill in 1642, was fought when the Royalists chose to move off an escarpment to a less advantageous position so that the Parliamentarians would be willing to fight.
Pitched battles may result from a meeting engagement, where--instead of disengaging--the opposing generals choose to reinforce their positions and turn what was initially a skirmish into a pitched battle, as happened in the 1863 Battle of Gettysburg during the American Civil War.
The last pitched battle on British soil was the Battle of Culloden in 1746.
Recreational battle reenactment tends to focus on pitched battles partially for the sake of ease of demonstration.