Wikipedia
In Tibetan Buddhism and Bon, a Ngakpa (; Sanskrit mantrī) is a non-monastic practitioner of Dzogchen who has received a skra dbang, a hair empowerment, for example in the Dudjom Tersar lineage. This empowers one's hair as the home of the dakinis and therefore can never be cut. The term is specifically used to refer to lamas and practitioners (male or female) who are “tantric specialists” and may technically be applied to both married householder tantric priests (khyim pa sngags pa) and to those ordained monastics whose principal focus and specialization is vajrayana practice. However, in common parlance (and many western books on Tibetan Buddhism), “ngakpa” is often used only in reference to non monastic Vajrayana priests, especially those in the Nyingma and Bonpo traditions.
In Bhutan, and some other parts of the Himalayas, the term gomchen is the term most often used to refer to this type of Vajrayana priest, with ngakpa being reserved only for the most accomplished adepts amongst them who have become renowned for their mantras being particularly efficacious.
Traditionally, many Nyingma ngakpas wear uncut hair and white robes and these are sometimes called "the white-robed and uncut-hair group" (Wylie: gos dkar lcang lo'i sde).