The Tillotson equation (or Tillotson equation of state
or Tillotson EoS) is an equation of state (relating
pressure, volume, and temperature) for the shock
of very high speed impacts of metals
(metals in the "chemistry" sense, e.g., iron).
It is an equation-form that includes heuristic parameters intended to
capture the relevant characteristics of the material,
in particular, those associated with the material's melting and/or
vaporization.
Planetary science uses the equation in some models
of impacts within the solar system.