A Polycyclic Aromatic Hydrocarbon (PAH) is a Hydrocarbon (i.e., compound of Hydrogen and Carbon), more specifically, an Aromatic Hydrocarbon (including a ring), arranged in benzene rings interconnected like chickenwire. There are many types of PAHs depending on how many such rings make up the molecule.
On Earth, PAHs are pollutants, some carcinogenic. The Infrared signatures of PAH Emissions have been observed in interstellar space, suggesting theories about how compounds are formed in Molecular Clouds. It has been suggested that PAHs are the building blocks of life, first created in such clouds.
At least some aromatic hydrocarbons, including at least some PAHs exhibit Fluorescence, which can be used to detect them. Such fluorescence might be a usable Biosignature usable in In Situ investigations, such as on solar system Moons and planets.