yo or yon or yrn
Yarn over, yarn over needle or yarn round needle is the same for all of these abbreviations. You wrap the yarn around the right hand needle creating an extra stitch that looks like an open space or a hole in your fabric. This is most commonly used in lace knitting. The most important thing to remember is that no matter how you bring the yarn around the needle, it must end up on the side of your next stitch. If your next stitch is a knit, the yarn must be at the back of your work. If your next stitch is a purl, the yarn must be at the front of your work.