Does it look like this? If so it is called a castle nut and a cotter pin passing through the bolt keeps it from loosening.

I thought they were called Castellated Nuts.

(For use with cotter pins, inserted through holes drilled through the bolts.)

Genuine Aircraft Hardware Co.

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Nuts, Castle

For a use with Drilled Bolts and a Cotter Pin

We have a large selection of Castellated Nuts or in Europe they may be called Crown Nuts.

I have seen single slotted nuts that could receive a screwdriver tip for.... backing purposes?

I think you will find the type of nut you are talking about is used in the automotive industry among others, this design has the thread slightly offset at the top, and when tightened it forces the slotted part outwards thereby causing it to lock on to the bolt.

They are sometimes referred to as prevailing torque nuts.

Further to my last posting ref; the nut with a slot, these were called "binks" nuts I think. best wishes.

I have seen those called "aero nuts". Have not got a clue why but that is what they were called in the manual of this American printing machine.

Used for vibrating parts like solenoinds which had to stay put.

Those nuts, whatever name is given to them and whatever form they take (one slot clinching, three slots clinching, twelve point deformed end, punch deformed on one flat...), are extremely commonly used on aircraft, particularly engine / turbine assembly and accessory attachment. I wouldn't be surprised if the history of their development is directly tied to the aircraft industry going way back.

Further to the J-man, the ultimate purpose is to adjust the nut to a specific torque and then retain it at that torque without it 'backing off'. The most common applications are in automotive front ends, where they're used for retaining tie rods, king pins, etc., but the easiest application to understand is the preloading of tapered bearings, front wheel bearings being again the best example.

It provides a way for a split pin to be inserted through a hole through the threaded part of the assembly and between the castellation pieces, in order to reduce the risk of the <...nut...> working loose once the split pin is opened out.