Originally the belief that a meltdown of a nuclear facility would result in such extreme temperatures that the reactor core would melt clean through the earth, coming out on the other side i.e. China. Can also be used to describe any catastrophe.
Wow that film last night was a bad case of china syndrome
Homer: But Mr. Burns gave me my job, and he hasn't fired me even after three meltdowns and one China Syndrome! I can't betray him!