Stitch Holder
A stitch holder is a large, lockable safety-pin-like tool or a strand of waste yarn that holds live stitches safely while they wait to be knitted again.
Explanation
A stitch holder is used to temporarily set aside a set of live stitches that won't be knitted right away, such as the stitches for a sleeve cap or a pocket opening, while you continue knitting the rest of the garment. The most common options are a purpose-made stitch holder that looks like a large safety pin, or simply a long strand of waste yarn threaded through the stitches. A strand of waste yarn is often preferred for many stitches or longer breaks, since stitches can't slide off the way they can from a holder.