A term used in astrophysics, the critical circumference is a limit to the size an object of a given mass can be, without imploding to form a black hole. It's given by the equation :-

Critical Circumference = 18.5 km x Mass

Where the mass is in solar masses. So if a mass the size of the sun ever shrinks so it's circumference is less than 18.5 km, it's event horizon time!

This value is needed for the hoop conjecture, to model the collapse of non-spherical masses.