The virial parameter is a quantity calculable from the mass, radius and velocity dispersion of a system such as a molecular cloud or dense core, that demonstrates whether it is gravitationally bound. It is essentially the ratio of the cloud's kinetic energy to gravitational potential energy.
5σ² α = —— GM
If α >> 1, the system is not gravitationally bound, and must be held together by external pressure. If gravitationally bound, α ~ 1.
This relation is commonly used to deduce the mass of molecular clouds.