What is Foreign Body?

Legal Definition
A foreign body or sometimes known as FB (Latin: corpus alienum) is any object originating outside the body. In machinery, it can mean any unwanted intruding object.

Most references to foreign bodies involve propulsion through natural orifices into hollow organs.

Foreign bodies can be inert or irritating. If they irritate they will cause inflammation and scarring. They can bring infection into the body or acquire infectious agents and protect them from the body's immune defenses. They can obstruct passageways either by their size or by the scarring they cause. Some can be toxic or generate toxic chemicals from reactions with chemicals produced by the body, as is the case with many examples of ingested metal objects.

With sufficient force (as in firing of bullets), a foreign body can become lodged into nearly any tissue.
-- Wikipedia
Legal Definition
An extra body in food making it contaminated and unfit.