A split beam nut, also known as a split hex nut or slotted beam nut, is a locknut

Locknut

A locknut, also known as a lock nut, locking nut, prevailing torque nut, stiff nut or elastic stop nut, is a nut that resists loosening under vibrations and torque. Elastic stop nuts and prevailing torque nuts are of the particular type where some portion of the nut deforms elastically to provide a...

 with slots cut in the top that separate the outside end into two or more sections that are bent slightly inward, making the thread diameter undersized in the slotted portion. As the nut

Nut (hardware)

A nut is a type of hardware fastener with a threaded hole. Nuts are almost always used opposite a mating bolt to fasten a stack of parts together. The two partners are kept together by a combination of their threads' friction, a slight stretch of the bolt, and compression of the parts...

 is threaded on, these sections are forced back out to their original position and increase the friction between the nut and the fastener, creating the locking action. High strength grades retain their locking ability up to 1400 °F (760 °C), unlike nyloc nut

Nyloc nut

A nyloc nut, also known nylon insert lock nut, polymer insert lock nut or elastic stop nut , is a kind of nut that includes a nylon collar insert. The insert is placed at the end of the nut and its inner diameter is slightly smaller than the major diameter of the screw...

s. Military grade nuts can be reused at least a dozen times.

Aerotight is a tradename for a specific type of split beam nut.