Definition - What does Natha mean?
Natha is a Sanskrit term that can be translated to mean "lord," "master" or "protector." It is most often used to refer to the nine naths (saints/gurus) of the Nath tradition. The Nath tradition is a heterodox siddha (adept) tradition that was established by Matsyendra and further developed by his disciple, Goraksha. These two figures were also the founders of Hatha yoga.
The Nath tradition has many sub-sects, but they all accept Matsyendra and Goraksha as their founders. The Natha Sampradaya, or the "Tradition of the Nine Gurus," is based on a lineage of spiritual masters who are worshiped both collectively and individually.
The foundation and wisdom of the Natha Sampradaya tradition is based in the principles of yoga and, therefore, great yogis are sometimes called nathas.
Yogapedia explains Natha
The term adi natha (or adi-nath) translates to "the first lord." Many in the Natha Sampradaya tradition believe this to be Lord Shiva himself. He is both the accepted patron god of the Nath sect and the Lord of yoga, or the Yogesvara.
The Natha yoga tradition has been very influential all over India, mostly in the North and West. Originally, the central aim of this practice was to achieve moksha (spiritual liberation) or jivanmukti (Self-realization) during the practitioner's lifetime. Those in the Natha Sampradaya believed that doing so did not depend on the benevolence of the Divine, but on an individual's own thinking and action.
The establishment of the Natha Sampradaya as a distinct sect is dated around the 8th or 9th century C.E., beginning with a simple fisherman and his disciple. Today, the ideas introduced by Natha Goraksha are still being implemented by millions of people all over the world.