A vent is a vertical slit rising from the bottom hem of a jacket or a skirt, generally to allow for ease of movement.
In the case of jackets, vents were originally a sporting option, designed to make riding easier, so are traditional on hacking jackets, formal coats such as a morning coat, and, for reasons of pragmatism, overcoats. Today there are three styles of vent: the single-vented style (with one vent, either directly at the center or roughly 3 cm to the right); the ventless style; and the double-vented style (one vent on each side). Vents are convenient, particularly when using a pocket or sitting down, to improve the hang of the jacket,  so are now used on most jackets. Ventless jackets are associated with Italian tailoring, while the double-vented style is typically British. (This is not the case with all types of jackets. For instance, dinner jackets traditionally take no vents.)
引用元：Vent (tailoring) – Wikipedia https://en.wikipedia.org/wiki/Vent_(tailoring)