Transmission spectroscopy is study of the difference in the spectrum of electromagnetic radiation passing through a substance and the source electromagnetic radiation. The term transmission spectroscopy is used in extra-solar planet research but the meaning is pretty much the same as absorption spectroscopy, as opposed to the study of spectrums emitted by a substance (emission spectroscopy) or the effect of reflection on a spectrum (reflection spectroscopy).
Transmission spectroscopy is useful when some of the radiation of a source (or multiple similar sources) passes through a gas and other does not. By comparing the two, a spectrum from the gas is evident. This assists in studying gas and plasma in space, and is very helpful in studying the atmosphere of exoplanets as well as planets and moons of the solar system.