Nomenclature in astronomy is often very confusing if not intentionally obfuscating. Every subfield arbitrarily introduces its own symbols and a book encompassing many different subfields has to dance a fine line between consistency with common usage in the literature and avoiding internal confusion. I have tried to avoid using the same symbols for more than one quantity as much as possible but did not quite succeed. This glossary provides a list of symbols that are used throughout the book, often with a reference to the equation where the symbol is first introduced or best defined. In those cases where a symbol is used in a different way than listed here – and this is not obvious from the context – I have tried to redefine the symbol consistently in the text.

• Aij Einstein A coefficient (Eq. 2.17)

• Aj The abundance of species j (Eq. 2.37)

• Av The dust extinction at visual wavelength, in magnitudes

• Aλ The dust extinction at wavelength λ, in magnitudes (Eqs. (5.2) and (5.84))

• a The grain size

• B(ν, T) The Planck function at frequency ν and temperature T

• Bij Einstein B coefficient (Eq. (2.18))

• Be (also B, A, C) Rotational constant (Eqs. (2.4), (2.8), (2.9), (2.11))

• B0 The preshock magnetic field strength

• B1 The postshock magnetic field strength

• b The Doppler line-width parameter

• CV The heat capacity of the grain material

• Cs The speed of sound (Eq. (11.8))

• CI The speed of sound in neutral gas

• CII The speed of sound in ionized gas

• c The speed of light […]