Is it ensure, insure, or assure?
c.1600, from French cathéter, from Late Latin catheter "a catheter," from Greek katheter "surgical catheter," literally "anything let down," from stem of kathienai "to let down, thrust in," from kata "down" (see cata-) + stem of hienai "to send" (see jet (v.)). Earlier was cathirum (early 15c.), directly from Medieval Latin. Related: Catheterization; catheterized; catheterizing.
catheter cath·e·ter (kāth'ĭ-tər)
A hollow, flexible tube inserted into a body cavity, duct, or vessel to allow the passage of fluids or distend a passageway; its many uses include the diagnosis of heart disorders when inserted through a blood vessel into the heart.