invagination

Invagination describes the act of one part folding inward or being drawn into another part of the same structure. It is used most often in biology and medicine to describe a surface, tissue, or membrane that turns inward, creating a pocket, channel, or tube-within-a-tube form.

To picture it, think of pushing the finger of a glove inward so that it turns into the glove itself. Unlike a simple bend or pull, this kind of inward folding creates a new inner space. A related medical example is intussusception, where one part of the intestine slides into the next section in a telescoping way.

In biology, this inward folding is a key part of embryonic development. Early groups of cells fold inward to help build the basic body plan and form structures that later develop into organs and other internal parts. The idea is simple but important: an outside layer moves inward and helps shape what comes next.

The word can also be used more broadly for similar inward-folding shapes in other contexts, but its main home is scientific writing. Whether it describes a cell surface folding in, an early embryo forming new layers, or a section of bowel sliding inward, the central idea stays the same: an outer part turns inward and forms a pocket or nested structure.