What is the term for a long flag or an open tapering?

The term described as a long flag or an open tapering is commonly referred to as a "banner." A banner typically represents a piece of cloth with insignia or logos and is often used for display purposes, particularly at events or gatherings. It can taper at the ends and is designed to be visually striking, thus aligning with the description provided in the question.

In contrast, the term "waft" usually refers to the action of carrying something lightly through the air, especially a scent, and doesn’t relate to flags or tapering shapes. "Stream" generally denotes something flowing, like water or gas, and does not pertain to flags. "Furl" refers to the action of rolling up or folding a flag, which again doesn't match the description of a long flag or tapering form. Therefore, "banner" is the term that best fits the criteria laid out in the question.