(covers information from several alternate timelines)
A foreign body or foreign substance was any material which originated from somewhere other than the space it occupied. The term was often used in medical terminology to describe an infection, such as a parasite, or an object, such as a splinter, originating outside the body of an organism.
In 2367, Dr. Beverly Crusher theorized that the presence of a foreign body inside Susanna Leijten could explain why a T-cell stimulator was ineffective in halting her transformation. (TNG: "Identity Crisis")
In 2375, The Doctor suggesting making USS Voyager unpalatable to the telepathic pitcher plant, allowing them to escape. He reasoned that it would work because "any living organism, no matter how large, will attempt to expel a foreign body." (VOY: "Bliss")