ingression

Ingression is a noun meaning the act or process of entering. The term comes from Latin ingressus ("a going in"), derived from ingredi ("to enter"). It is used in a range of disciplines to describe a boundary-crossing event or movement into a space or domain.

In developmental biology, ingression refers to a specific cell movement during embryogenesis. Individual cells break away from an epithelium and migrate inward into the underlying tissue, a process associated with epithelial–mesenchymal transition. Ingression is one pathway by which tissues reorganize during gastrulation and can contrast with other movements such as invagination (folding of a layer) or involution (inward spreading of a tissue sheet).

In astronomy and planetary science, the preferred term for the beginning of a transit is ingress; ingression is rare and usually considered a less standard variant. When used, it often carries the same meaning as ingress but may appear in older texts or in broad discussions of boundary entry events.

To avoid confusion with the video game Ingress, unrelated to the astronomical or biological senses, the two terms are distinct in usage. Ingression remains a general and specialized term whose exact meaning depends on the disciplinary context.