spectrum: The splitting of, say, an electromagnetic wave into its component frequencies.
spin: An internal property of elementary particles, related to, but not identical to, the everyday concept of spin.
stationary state: One that is not changing with time: a sphere spinning at a constant rate is stationary because it looks identical at any instant, even though it is not static.
steady state theory: A theory of cosmology, now largely discredited, in which new matter is formed in the expanding space between existing galaxies.
