PICC = peripherally inserted central catheter. A long intravenous line inserted usually in the arm which passes along a vein and ends up in one of the big veins in the thorax, used for mid- to long-term (weeks) IV treatments including antibiotics, chemotherapy, and intravenous feeding.
Compare it to other forms of IV access through the arm, via a standard Venflon™ cannula, which are only really guaranteed for about 3 days at a time before they need changing, but are otherwise very easy to insert.
Also compare it to long-term central venous access required for several months, such as Hickman lines for long-term immunosuppressants or chemotherapy (e.g. for certain inflammatory neurological disorders) or central haemodialysis access in the neck or groin.