Four Cycles of Nyingtig

Tib., snying thig skor bzhi

The chief disciple of Manjushrimitra, the great master known as Sri Singha, divided the Instruction Section of the Nyingtig teachings into four 'Cycles'. Corresponding to these four cycles, there is also the expression snying thig gi dbang bzhi, referring to the four initiations (empowerments) associated with these teachings.

Outer Cycle
Inner Cycle
Secret Cycle
Innermost Unexcelled Cycle (Tib., yang gsang bla na med pa'i snying thig gi skor).

The last section is mostly said to contain the 17 major tantras, yet there are 18 when (sometimes) the Ngagsung Tromay Tantra (focused on protective rites of the goddess Ekajati) is added; and even 19 according to the system of Padmasambhava; which includes the Longsel Barwey Tantra (Tantra of the Blazing Space of Luminosity).

This fourth section is also known by the short name Secret Heart Essence (gsang ba snying thig), actually a synonym for Dzogchen (rdzogs pa chen po). In Tibetan texts as well as in various translations, a variety of Tibetan terms and English renderings are in use for this fourth cycle:

Most Secret Unexcelled Nyingtig (yang gsang bla na med pa snying tig)
Innermost Unexcelled Cycle of Nyingtig (yang gsang bla na med pa'i snying thig skor)
Most Secret and Unexcelled Great Perfection (yang gsang bla na med pa rdzogs pa chen po)
Most Secret Heart Essence (yang gsang snying thig)
Most Secret Unsurpassable Cycle (yang gsang bla na med pa'i sde)