Absorption spectrum
Also called Dark line spectrum
An absorption spectrum is a continuous spectrum crossed by dark lines, produced when a substance removes the photons whose energies match its own energy level differences. The pattern of gaps identifies what the light passed through.
Send continuous light through a gas and the gas removes photons whose energy matches a gap between two of its energy levels, lifting atoms from a lower state to a higher one (15.3.A.2.i). What emerges is a continuous spectrum with narrow dark lines where those wavelengths used to be.
Those dark lines fall at the same wavelengths as that element's bright emission lines, because the same pairs of levels are involved either way. Only the direction of the transition and the appearance change: dark lines on a continuous background here, bright lines on darkness for emission.
That is the whole distinction, and it is the one students state backwards under time pressure. Absorption removes light from a beam passing through a substance. Emission adds light from an excited source.
Essential knowledge 15.3.A.4.ii gives the application: an absorption spectrum can be used to determine the elements composing a substance by observing what light the substance has absorbed. Because 15.3.A.4 makes each element's set of frequencies unique, the pattern of gaps is an identification and not merely a measurement.
Any diagram you are handed will be for a single-electron atom, by the Topic 15.3 boundary statement.