'spectral' originates from Latin via Medieval Latin, specifically from the noun 'spectrum' (Latin) meaning 'appearance, image' and the adjective-forming suffix '-al'; the Latin root comes from 'specere'/'spec-,' meaning 'to look' or 'to see'.

'spectral' developed from Medieval Latin 'spectralis' (formed from 'spectrum') and passed into modern European languages (e.g., Middle French) before being adopted into English as 'spectral.'

Initially associated with 'appearance' or 'apparition' (hence 'ghostly'); over time it also acquired a technical sense 'relating to a spectrum' in optics, physics, chemistry, and mathematics.