Disc springs or Belleville washers (also refered to Belleville Springs,Conical Washers,Spring Washers,etc.), a non-flat washer with a conical shape which gives the washers their spring characteristics and are designed to be loaded in the axial direction only. Typically used as springs or to apply a preload with a flexible quality to a bolted joint.
Disc springs used singularly or stacked in different combinations allows for variable spring characteristics suiting the given application. (from Reason Technology Company)(DIN 2093)