Under bust measurement - A measurement taken around the rib cage, just under the bust.
Under collar - The facing portion of the collar that typically rests against the garment.
Underlining - Underlining is a fabric that is cut to match the garment fabric and is sewn together with the garment fabric as a unit during garment construction. Underlining is used with lightweight fabrics to give them strength, to reduce wrinkling, and in cases of transparent or light colored fabrics, to make them more opaque. Underlining can also be used to conceal hand hemming (or other hand stitching) because the hand stitches are taken into the underlining fabric rather than the garment fabric itself.
Understitching - A line of stitching that secures the facing to the seam after the facing has been pressed away from the garment. This helps keeps the facing from rolling up and becoming visible on the garment while it is worn. My mother swore by this and always made me do it. And, you know, she was right. It really does make the facing lay nicer.
Upper bust measurement - This measurement is taken around the chest under the arms and above the bosom.
Upper collar - This is the visible element of the collar that faces out from the garment.
Walking foot - This is a specialized foot that can be purchased if one does not come with your machine. It regulates the pressure of the foot at the needle, helping to move fabric under the foot evenly. The feed dogs below the plate can sometimes move the lower fabric along faster than the top fabric, especially if the two fabrics are different from each other. A walking foot is especially helpful when sewing on slippery or thin fabrics. I swear by mine and use it all the time.
Warp - The long loom threads that go the length of the fabric, parallel to the selvage edge. They are stronger than the weft threads that go side to side which is why patterns tend to cut following the length-wise grain.
Wearing ease - This refers to the amount of ease built into a pattern that alllows the resulting garment to be worn comfortably on the body.
Weft - The threads that run across the width of the fabric between the selvages. Also called a crosswise grain.
Wing needle - A sewing machine needle with flared sides (wings) that makes holes in fabric as it passes through it, by spreading the threads to the sides. A wing needle can be used in combination with specialized stitches such as honeycomb or eyelet to create decorative openwork stitches.
Wonder-Under - This is a double-sided fusible product made by Pellon that allows layers of fabric to be permanently fused together applique fashion by using an iron. The raw edges of the fabric can be left unfinished or can be appliqued using a zigzag stitch or covered with fabric paint.
Yoke - A flat panel section that divides a garment horizontally, most frequently used at the shoulder or waist. A common example of a yoke can be seen on the back of men's dress shirts across the shoulder.
Zig-zag - A type of sewing machine stitch that goes side to side as it goes forward. The width of the stitch can be altered as well as the length to suit various applications. Raw fabric edges can be zig-zagged to prevent fraying in the absence of a serger. Butonholes are made using a narrow zig-zag stitch. When the stitch length is set to almost zero, it creates a satin stitch that can be used as a decorative seam finish for applique.