Definition of phacolith in US English:

phacolith

noun

Geology
  • An intrusive mass of igneous rock, typically lens-shaped, situated between consecutive strata at the top of an anticline or the bottom of a syncline.

Origin

Early 20th century; earliest use found in Philip Lake (1865–1949). From phaco- + -lith.

Pronunciation

phacolith

/ˈfakəlɪθ/