Light weight or sport yarn. Sometimes referred to as Light DK. Yarns include 3ply, 4ply, 5ply and 6ply yarns. Needle sizes 2.5-3.75mm. This is the perfect yarn for sock knitting, lace knitting, light summer knitwear or knitting for babies.
Please note that "ply" is a rather imprecise description of yarn weight. Always check the required tension/gauge if choosing a yarn for a particular pattern.