Highly oriented pyrolytic graphite (HOPG) is a highly pure and ordered form of synthetic graphite. It is characterised by a low mosaic spread angle, meaning that the individual graphite crystallites are well aligned with each other. The best HOPG samples have mosaic spreads of less than 1 degree.
Note that the term "highly ordered pyrolytic graphite" is sometimes used for this material, but IUPAC favors "highly oriented".[1]