Term that describes a state where the system is stable, but any small perturbation
will cause it to decay. A straightforward example is a ball sitting at the top of a mound. Right at the center of the mound, it is quite stable. If a gust of wind were to blow it in any direction, then it would quickly begin to roll down the side of the mound until it reached the bottom.
Metastable states are important in physics and chemistry because they represent intermediates which may live long enough to do interesting things. The non-infectious form of a prion is a metastable protein. However, once a metastable prion comes into contact with a disease prone prion, it too becomes quickly converted. For more details, see the writeup under prions.