An instrument for withdrawing fluid from a cavity, or for use in paracentesis. It consists of a metal tube (cannula) into which fits an obturator with a sharp three-cornered tip, which is withdrawn after the instrument has been pushed into the cavity; the name trocar is usually applied to the obturator alone, the entire instrument being designated trocar and cannula. [Fr. trocart, fr. trois, three, + carre, side (of a sword blade)]