phac·o·lith \ˈ ̷ ̷ ̷ ̷ˌlith\noun (-s) Etymology:phac- + -lith : a lens-shaped mass of igneous rock intruded in folded sedimentary beds with which it is approximately concordant and having its greatest thickness along the axes of synclines or anticlines • phac·o·lith·ic\| ̷ ̷ ̷ ̷|lithik\adjective