The term diffuse emission is used in astronomy/astrophysics for emitted electromagnetic radiation from a larger region of the celestial sphere than a point source, i.e., from a (presumably resolved) structure with some extent, or from much or all of the whole sky (the word diffuse basically means not concentrated).
The phrase diffuse emission is used both for Milky-Way and extragalactic emissions. Nebulae (or some of them) have been referred to as diffuse nebulae. The term diffuse emission nebula has been used for nebulae showing an emission line, and is sometimes used to refer to a HII region.
Any part of the cosmic background radiation may be referred to as diffuse emission, including the CMB, as well as the emission lines that are the subject of cosmology experiments such as intensity mapping, and surveys aimed at star formation history. The term has also been used for the X-rays from the plasma spread through galaxy clusters