The characterization of the aging of electric motors using vibration measurements and a geometric interpretation of spectral domain signatures is presented. For comparison, two vibration signal records from a three-phase induction motor were acquired during degradation due to bearing fluting, following thermal and chemical aging sequences. Power spectral densities (PSD) of the vibration signals were calculated and linear approximations to the spectral signatures were used to observe the overall trend. The intersection point and slopes of these linear representations were used to extract certain critical frequencies in order to define convex regions (CR) and convex hulls (CH) as representatives of the aging (degradation) process. Furthermore, using these CRs and CHs, the functional readiness of motors could be determined by comparison of the signatures for initial and aged conditions.