YO (Yarn Over)
YO means wrapping the yarn over the needle before the next stitch, creating both a new stitch and a small hole. Fundamental to lace knitting.
Explanation
YO (yarn over) is worked by bringing the yarn over the needle before knitting the next stitch, without knitting a stitch out of the wrap itself. This increases the stitch count by one while creating a small, decorative hole in the fabric. YO is the basic building block of lace knitting and many decorative patterns, and is almost always paired with a matching decrease (like k2tog or ssk) elsewhere in the round to keep the total stitch count stable.