# Definition question: continuum-subtracted spectrum

What is the exact definition of a "continuum-subtracted spectrum", such that one finds in SDSS spectra?

Is there a definition somewhere? Thanks

In the case of galaxies, the continuum consists of the combined spectrum of many different stars' different blackbody spectra, infrared light from dust, etc. On top of this, there are lines, e.g. from nebular processes such as my favorite line Lyman $\alpha$, which is emitted when hard UV radiation from hot O and B stars ionize their surrounding neutral hydrogen, which subsequently recombines emitting mostly Lyman $\alpha$.
In the top figure below, the total spectrum's flux density$^\dagger$ is in black, and the fit is the red lines. Underneath is drawn the continuum-subtracted spectrum, and a few dashed green lines show some lines that rise above the continuum. The integrated line flux is indicated by light red.
$^\dagger$Note that many authors use the term "flux" where it should actually be "flux density". Integrating flux density in $\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{cm}^{-2}\,\mathrm{Å}^{-1}$ over wavelength in Å$^{-1}$ gives flux in $\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{cm}^{-2}$.