Is your feature request related to a problem? Please describe.
The user guide definition of variables and symbols section (found here) does not currently have a standard definition for "spectral irradiance".

Describe the solution you'd like

• Agree a standard definition for spectral irradiance as a variable
  I think having a standard "root" variable that can have super/subscripts added to for function-specific requirements would be good, for example, something like ghi / ghi_extra, aoi / aoi_projection , or temp_air / temp_cell / temp_dew (the "root" here being ghi, aoi, and temp)

• Add the definition to the user guide

• Update existing code if required

Additional context

• pvlib.spectrum.calc_spectral_mismatch_field takes e_sun and e_ref as arguments for field and reference spectral irradiance, respectively.
• pvlib.spectrum.spectrl2 and pvlib.spectrum.get_reference_spectra both use spectra inside the code (no spectral irradiance user-facing argument)
• pvlib.spectrum.average_photon_energy (PR) could also benefit from this

Suggestions: (updated from comments)
si, (si_sun, si_ref...)
spectral_irr
e
spectrum
spectra
spectral_irradiance
_spectral (suffix, e.g. ghi_spectral)

e_sun and e_ref make sense in calc_spectral_mismatch_field() but, for a function that doesn't require the _sun/_ref specification, e alone might be ambiguous. I like spectral_irr as it's clear, but it is potentially too long? A shorter alternative is si.

Thoughts? Other suggestions?