What term refers to the external opening of a tube in the human body?

The term that refers to the external opening of a tube in the human body is "meatus." This anatomical term specifically describes the opening of a canal or duct, such as the urinary meatus, which is the external opening of the urethra. The meatus serves as an important passageway for bodily fluids and plays a significant role in the functioning of various systems.

Considering the other terms: "gastrocnemius" refers to a muscle in the back part of the lower leg; "stoma" is a term that can refer to any opening but is often associated with surgically created openings in the body, such as in colostomy procedures; "fissure" denotes a narrow slit or opening, often associated with abnormal or pathological conditions in the body. While these terms describe different structures or openings, they do not specifically identify the external opening of a tube, which makes "meatus" the most accurate choice.