These are known as a Dome Nut or Cap nut in Ireland and are called an acorn nut in the USA. They are a popular nut used in applications where aesthetics are important, giving an attractive finish. These are dome Shaped and they ensure a smooth snag-free surface protecting both the fixing and also reduces the effects of physical mishaps on human skin if it somehow forcefully hits the fasteners.
These nuts are manufactured to DIN 1587.It is available in stainless steel, carbon steel, and alloy steel to bear more wear and tear while also making it more corrosion resistant.
Also available are Dome Nyloc Nut manufactured to DIN 986. Available in UNF, UNC and Metric sizes.