Definitions for "Grain Boundary"
A grain is a a portion of a solid metal (usually a fraction of an inch in size), in which the atoms are arranged in an orderly pattern. The irregular junction of two adjacent grains is known as a grain boundary. This narrow zone corresponds to the transition from one crystallographic orientation to another, thus separating one grain from another.
A surface defect representing the boundary between two grains. The lattice has a different orientation on either side of the grain boundary. Interstitialcy.  Atom occupying an interstitial site not normally occupied by an atom in the perfect crystal structure or an extra atom inserted into the perfect crystal such that two atoms occupy positions close to a singly occupied atomic site in the perfect structure.
a thin layer of atoms located in between two separate crystals or grains of a polycrystalline material.
Keywords:  demarcation, line
The line of demarcation between grains.