spontaneous symmetry breaking

Spontaneous symmetry breaking is said to occur when the theory that describes a system contains a symmetry that is not manifest in the ground state. A simple everyday example is a pencil balanced on its tip. The pencil, which is symmetric about its long axis and equally likely to fall in any direction, is in an unstable equilibrium. If anything (spontaneously) disturbs the pencil, it will fall over in a particular direction and the symmetry will no longer be manifest.