Lifeline
A lifeline is a strand of smooth waste yarn threaded through a full round of stitches, making it safe to frog back to a known point if you lose your place.
Explanation
A lifeline is a thin, smooth strand, often in a different material than the project yarn, threaded through every stitch in a full round using a tapestry needle. If you later discover a mistake further up in the work, you can safely frog the knitting down to the lifeline without risking stitches falling off entirely and tangling, since the lifeline holds the stitches in place at that point. The technique is widely used in complex lace and cable patterns, where it's easy to lose track and a mistake can be hard to spot until many rows later.