A weft is a basic component used in weaving to turn thread or yarn into fabric. It is the horizontal component of the weave, while the lengthwise or longitudinal component is called the warp. The warp yarns are held stationary in tension on a frame or loom while the weft is drawn through and inserted over and under the warp. A single thread of the weft crossing the warp is called a pick. Terms vary, for instance, in North America, the weft is sometimes referred to as the fill or the filling yarn.

In the context of hair extensions, a hair weft is a collection of hair strands sewn onto a super-thin cloth strip ready for a stylist to work into a clients hair. The weft can be machine-sewn or sewn by hand, and it can be made of real or synthetic hair. The weft is attached to the clients natural hair using various methods, including bonding, sewing, clips, and tape-ins. Weft hair extensions are a popular choice for adding length, volume, or color to natural hair, and they can last for weeks or even months at a time.