Definition of megacryst in US English:

megacryst

noun

Geology
  • In an igneous or metamorphic rock: a crystal that is significantly larger than those in the surrounding groundmass or matrix.

Origin

1950s. From mega- + cryst-, after phenocryst, metacryst.

Pronunciation

megacryst

/ˈmɛɡəkrɪst/