The Nyingma tradition is the oldest of the four major schools of Tibetan Buddhism (the other three being the Kagyu, Sakya and Gelug). "Nyingma" literally means "ancient," and is often referred to as Nga'gyur (Tibetan: སྔ་འགྱུར།, Wylie: snga 'gyur, ZYPY: Nga'gyur, school of the ancient translations) or the "old school" because it is founded on the first translations of Buddhist scriptures from Sanskrit into Tibetan, in the eighth century. The Tibetan script and grammar was actually created for this endeavour. In modern times the Nyingma lineage has been centered in Kham in eastern Tibet.
Read more about Nyingma: Early Lineage and Traditions, History, Tantra and Dzogchen Texts and Praxis in The Nyingma Tradition, Termas and Tertons, Various Traditions and Important Historical Figures, Contemporary Lineage Teachers