Spring Washer, these are a thin circular plate with a central hole but have a split in and bent into a helical shape. They are used to increase the force between the male and female thread increasing friction and therefore resistance to rotation, hence they are sometimes referred to as lock washers.
Form A : Serrated Lock washers
BZP - Bright Zinc Plated
T304 (A2) Stainless Steel
T316 (A4) Marine Grade Stainless Steel