Crossguards are a section of material, usually but not always, metal that crosses the blade near the hilt form a guard. Each projection from either side of the sword is known as a quillion.
It acts as a defensive structure against blades runing the length of a weapon and also may be used in an offensive manner.
The individual quillions may exhibit a mirror symmetry through the axes of the blade or they may have very different shapes. These shapes include the drop and bat wing.
A cruciform sword describes a sword with a straight crossguard that forms a cross when viewed point down. This is believed to be symbolic.
Other forms of guard include the basket hilt, and the rondel the latter being used on a variety of weapons.
In the SCA the crossguard is generally referred to as quillions.