1. n. 
A type of woody kerogen that is relatively uniform in composition. Since vitrinite changes predictably and consistently upon heating, its reflectance is a useful measurement of source rock maturity. Strictly speaking, the plant material that forms vitrinite did not occur prior to Ordovician time. Also, because vitrinite originated in wood, its occurrence in marine rocks might be limited by the depositional processes that act in a given depositional environment.