  A tachyon is considered to be sick in the context of particle mechanics, but
in field theory just indicates instability of a background. We consider a
similar possibility that a ghost in field theory might be just an indication of
instability of a background and that it can condense to form a different
background around which there is no ghost. We construct a low energy effective
field theory based on the derivative expansion around the stable background.
Possible applications are discussed, including dark energy, dark matter,
inflation and black hole.
