Spin foam - Definition and Overview

In physics, a spin foam is a four-dimensional graph made out of two-dimensional faces that represents one of the configurations that must be summed to obtain Feynman's path integral (functional integration) describing the alternative formulation of quantum gravity known as loop gravity or loop quantum gravity.

A spin foam may be viewed as a history of evolution of a spin network. Much like the more general term quantum foam, spin foam also bears a superficial resemblance to the old concept of the ether (or aether). For example, it generally breaks Lorentz invariance.