An Emission Line Galaxy (ELG) is a class of galaxies characterized by the presence of Emission Lines. The label is used for distant galaxies (i.e., with significant Redshift) that fit the description. Other classifications are Absorption Line Galaxies and A+E Galaxies (featuring both).
Emission lines are indicative of Star Formation, e.g., a Starburst Galaxy. Emission lines can be fortunate in assisting with redshift determination, e.g., confirming the distance to some of the most-distant/oldest galaxies.
Some typical emission lines are:
The term Extreme Emission Line Galaxy (EELG) is also used, for those with especially bright emission lines.