"spectral" refers to something related to a ghost or a spirit. It could be used to describe things that are mysterious, supernatural, or eerie in nature. Full definition
[94] Proxima Centauri b was found using the radial velocity method, where periodic Doppler shifts of spectral lines of the host star suggest an orbiting object. (en.wikipedia.org)
Ma, Q., R.H. Tipping, G. Birnbaum, and C. Boulet, 1998: Sum rules and the symmetry of the memory function in spectral line shape theories. (pubs.giss.nasa.gov)
Comparing their spectra suggests that young L dwarfs with similar ages and temperatures can have different spectral signatures of youth. (arxiv.org)