biocompatibility
Biocompatibility describes how well a material coexists with the body without setting off a harmful reaction. A biocompatible material is a good house guest: it does its job, stays out of trouble, and does not poison the cells around it, provoke a damaging immune attack, or make blood clot where it should not. A material that fails this test, however clever, simply cannot be used inside a person.
The body treats anything foreign with suspicion. Immune cells investigate a new implant, and if they read it as a threat they wall it off in scar tissue, inflame the area, or attack it outright. Biocompatibility is the art of designing the material's surface and chemistry so the body either ignores it peacefully or, better still, welcomes its own cells to move in and integrate with it.
Crucially, biocompatibility is not a fixed property stamped on a material — it depends on the job. A metal that is perfectly safe as a hip joint might be a disaster as a heart-valve surface where blood clotting is the danger. So engineers always ask biocompatible for what, and where, and for how long, rather than treating it as a simple yes-or-no label.
Titanium is a star biocompatible metal: bone cells will actually grow right onto its surface and lock a dental or hip implant firmly in place, as if the body adopted it. That happy bond is exactly why titanium shows up in so many implants.
Biocompatibility is always relative to the use: safe as one kind of implant does not mean safe everywhere in the body.