Sinus
Part of speech: noun
Definition: A pouch or cavity in any organ or tissue, especially the paranasal sinus. An abnormal cavity or passage such as a fistula, caused by the destruction of tissue.
Fistula
Part of speech: noun
Definition: An abnormal connection or passageway between organs or vessels that normally do not connect.A tube, a pipe or a hole. (Rare except in derived adjective fistular.)