Final answer:
The double-lock stitch is used to secure a weft at either end or across the length of a weft/track in hair wefting or hair extensions.
Step-by-step explanation:
The stitch that is used to secure a weft at either end or across the length of the weft/track is known as a double-lock stitch. This technique is very common in creating hair wefts or tracks in the field of cosmetology, especially when applying hair extensions. The double-lock stitch is integral to ensuring that the added hair remains securely attached to the natural hair.