An Emission Line is a Spectral Line due to Emission of Electromagnetic Radiation (EMR) from a (thin) cloud of gas with enough Atomic Excitation that the relaxation of atoms to lower levels is a primary source of its radiation, as per Kirchhoff's Laws. Such lines are used to identify clouds and analyze their properties: in addition indicating a component of their makeup, Redshifted (z) lines help estimate distance or Radial Velocity.
By contrast, Absorption Lines are produced when the Continuous Spectrum produced by a hot body passes through a (relatively) cooler gas, which shows a preference to Absorption of the Wavelengths matching the differences in energy from one atomic excitement level to another.
Atacama Pathfinder Experiment (APEX)
Carbon Monoxide (CO)
Carbon Monoxide Mapping Array (COMA)
Emission Line Galaxy (ELG)
HII Region (HII)
Molecular Cloud Turbulence
Molecular Deep Field
Palomar Four-Filter Survey (PC)
Star Formation Rate (SFR)
Submillimeter Galaxy (SMG)
Tully-Fisher Relation (TFR)
T-Tauri Star (TTS)
Zeeman-Doppler Imaging (ZDI)