Weft, in reference to weaving, refers to the yarn or thread that zigzags left to right through the woven fabric. On a loom the weft is the yarn that is carried by the shuttle back and forth through the vertical warp threads.
Unlike warp threads which are kept under high tension on the loom, the weft is not kept under the same amount of tension and therefore does not have to be constructed out of as strong of a yarn as the warp threads. Traditionally weft thread was made out of wool, cotton or flax; though today weft is made out of any number of synthetic fibers or synthetic fiber blends. Other names used to describe weft thread include woof, fill and filling yarn.
Because the weft does not have to hold as much tension as the warp, much thinner and more delicate yarns can be used. When making silk ties, it is typically the weft that is made from a much thinner, more delicate, and softer silk. For regimental repp ties and repp striped neckties that are made out of fabric blends such as mogador or poplin, it is the weft that is silk and the warp that is either cotton or wool.