The Chinese name for transparent noodles made from mung bean starch is fensi. Written 粉絲 or 粉丝 and pronounced fěnsī in Mandarin, the name is commonly translated as bean threads or bean-thread noodles.
Fensi belong to the broader family of glass noodles, also called cellophane noodles. Their clear, glass-like appearance develops after cooking, when the starch becomes translucent. They are usually sold dried and soaked before being added to soups, stir-fries, hot pots, spring rolls, or cold salads.
Although mung bean starch is traditional and especially common in Chinese varieties, glass noodles can also be made from sweet potato, potato, tapioca, or other starches. This is why the English category is broader than fensi: not every glass noodle is made from mung beans.
The name also helps distinguish them from wheat noodles such as ramen and from buckwheat noodles such as soba. Rice vermicelli is another common mix-up; it is made from rice and usually remains white rather than becoming transparent.