Definition
Plain language
In layered filesystems, the moment a file from a read-only layer gets duplicated to a writable layer because something edited it.
As stated in the literature
In OverlayFS and similar union filesystems, the operation that copies a file from a lower read-only layer to the writable upper layer upon first write; with reflinks, only the modified blocks are physically copied.
Why it matters: It lets many agent sandboxes share a large read-only base while keeping per-sandbox modifications small and isolated.
For example, an agent edits a config file inside a container, and OverlayFS copies that single file up from the base image into the writable layer rather than touching the rest.