Stemma was the Latin word for a 'garland' or 'wreath', in reference to the garlands and wreaths worn as a symbol of office in Rome and ancient Greece. However, this word was also used for the scroll recording one's family tree, and figuratively to refer to one's pedigree. This has been co-opted into a number of modern-day technical jargons.
1. It is still used today to refer to the formal recording of the genealogy of ancient Roman families. The stemmata of Roman emperors and the like are of interest to historians, and are often available one-line.
2. The field of stemmatics is the ordering of ancient manuscripts into a 'genealogy', so that one can track changes to the text over time. This is particularly common in the textual criticism of the Bible and other works of which the originals have been lost, such as the Iliad and Canterbury Tales. In this case a stemma is the complete (proposed) genealogy of a specific work.
3. The French linguist Lucien Tesnière used the term stemma to refer to his parse trees. This term is no longer in popular usage, but is sometimes seen in the field of linguistics when referring to his works.
4. In biology, a stemma (AKA lateral ocelli) is a type of small, circular, simple eye present in many endopterygotous larvae, some adults of various orders of insects (including fleas, springtails, and Thysanura), and some myriapods. Each stemma has a lens, behind which lies a cluster of photoreceptor cells (a retinula). The lens is biconvex, and the body contains a vitreous or crystalline core. This differentiates them from eye spots which do not have lens or vitreous humour, and dorsal ocelli, which generally have more photoreceptors and are generally supplementary to the primary compound eyes.
The plural is stemmata.