what is the name of this nut, I need to find a way to loosen it .
From the top
It's a spanner nut. There's a special wrench to loosen and tighten it but putting a slotted screwdriver in the groove and tapping it with a hammer usually works. it's a double nut so loosen one first, then the other.
I find using the correct “c” spanner, in fact I would use two together to undo that as it means no hammering to possibly damage bearings etc.
This method of using two nuts is used to set a limit or stop position - if only one nut was used it could easily move. A "nyloc" nut may also move but the two nuts locked together do not move. Some nuts can have a set-screw inserted to prevent movement but that is less secure than the two nuts...
Called a c spanner because it looks like a c but has a tooth at the end to engage in the slot on the nut.