noun

1. Biol. the theory that the embryo, influenced by its internal and external environment, develops progressively by stages, forming structures that were not originally present in the egg
2. Geol. a type of metamorphism in which the mineral content of a rock changes as a result of external influences
3. Med.
   1. the appearing of secondary symptoms
   2. a secondary symptom

Etymology: ModL: see epi- & -genesis