Well,I don't know you,not your exposure to the engineering world, but it's a very common way to define the bits that whirl around .
https://www.sciencedirect.com/science/article/abs/pii/S0007850619300678
https://www.redviking.com/our-work/dynamic-test-systems-our-work/Helicopter-Dynamic-Component-Test-Stands_PJ24.html