Permanently deletes a custom field definition and all data stored under it: its options, every contact's saved value for the field, its list relationships, and its form placements. The id must be a custom field definition id, never a field option id; to delete a single dropdown option use DELETE /fieldOptions/{id} instead. Field ids and field option ids are separate id spaces: passing an option id here deletes whichever unrelated field happens to have that id, or returns 404 if none does. Field definition ids come from the id of each entry in the fields array of GET /fields; ids inside the side-loaded fieldOptions array are option ids and are never valid here. This delete cannot be undone. To clear one contact's value for a field without touching the field itself, use DELETE /fieldValues/{id}.