Terma teachings originate with Guru Rinpoche, also known as Padmasambhava, who concealed many sacred teachings in such a manner that they would be discovered at appropriate times in the future, when beings would be ready to receive and understand those particular teachings.
The hidden teachings themselves are called terma, and the individuals foretold by Guru Rinpoche’s prophesies who discover these hidden teachings are known as tertons.
There are many different kinds of Terma, such as earth treasures, water treasures, and so forth.
The Green Tara sadhana is what is called “thought terma,” or gong ter.
Among the 1,002 named tertons, there are 108 individuals who are said to be great tertons, or Ter Chen.
Supreme among all of these was Orgyen Dechen Chokjur Lingpa, who discovered the Green Tara Practice as a mind terma.
The sadhana arose from the expanse of his mind in the way that water can suddenly spring from a rock where there was no water before.
This mind treasure arose in a cave called Peme Shelpuk, the “Crystal Cave of Pema.”
— from Bardor Tulku’s “The Practice of Green Tara”