imperturbable C2+

/ˌɪmpɚˈtɜrbəbəl/ unable to be upset or agitated; calm

perturb C2+

/pɚˈtɝb/ to make anxious or unsettled; to disturb a system slightly

perturbation C2+

/ˌpɜrtɚˈbeɪʃən/ a disturbance or deviation from a normal or expected state