Fatigue Damage Spectra (FDS) and Extreme Response Spectra (ERS) are well-established tools for characterizing and evaluating the impact of vibration loads on mechanical structures. While this is particularly valuable in producing test-tailored specifications, their current application is limited to uniaxial analysis despite evidence indicating their inadequacy in representing real-world loads, which are often multiaxial. The proposed method offers a system-oriented approach to characterize and compare multiaxial environments in terms of their severity, independently or not of any physical structure. In addition, it introduces a generalization of the FDS and ERS, referred to as Fatigue Damage multi-Spectrum (FDmS) and Extreme Response multi-Spectrum (ERmS) respectively. These extensions allows for the consideration of multiaxial phenomena, such as correlation and modal proximity. Finally, this paper demonstrates through several case studies the method’s capability to capture phenomena not addressed by established uniaxial approaches.