A stitch forming device used to interloop the bottom thread with the needle thread on a chainstitch, overedge, or coverstitch machine.
A stitch-forming device used on chainstitch, overedge and coverstitch sewing machines to form a stitch.
Noun: It's the thingie in the serger that carries the threads that form the overlock part of the serger stitch. Usually there are two, upper and lower. There may be a third which forms the lower part of the chain stitch.