Horns and boosters (and sometimes transducers) have a stud which is used to attach them to the adjacent resonator. Transducers normally have a stack bolt that secures the piezoelectric ceramics. The axis which passes through the axis of these studs is called the stud axis.
For cylindrical resonators, the stud axis defines a potential axis of symmetry of the resonator.
In some cases, "stud axis" may be called "resonator axis".