"Maha Nikaya" (literal translation: Great Collection) refers to one of the two principal sects of modern Thai Buddhism. The other principle sect is Dhammayuttika Nikaya. Generally, the Dahmayutti Nikaya sect of Theravada Buddhism is more strict than the Maha Nikaya sect.
The identification of the Maha Nikaya as a single, discrete, entity is an innovation of the Thai state. After the founding of the Dhammayuttika Nikaya by the then-monk Prince Mongkut in 1833, decades later all recognized monks not ordained in the Dhammayuttika order were considered to be part of the 'maha nikaya', the 'great collection' of those outside the new Dhammayuttika fraternity. As such, most monks in Thailand belong to the Maha Nikaya more or less by default; the order itself did not originally establish any particular practices or views that characterized those adhering to its lineage. There were in reality hundreds of different Nikayas throughout the Thai areas that were lumped together as the "Maha Nikaya".
In Cambodia, a similar situation exists; the Dhammayuttika Nikaya was imported from Thailand in 1855, and those monks remaining outside the Dhammayuttika order were recognized as being members of the 'Maha Nikaya' (Khmer: មហានិកាយ Mohanikay). A separate supreme patriarch for the Dhammayuttika Nikaya was appointed by King Norodom; the previous national supreme patriarch then became the titular head of the Cambodian Maha Nikaya.
In Thailand, a single supreme patriarch is recognized as having authority over both the Maha Nikaya and the Dhammayuttika Nikaya. In recent years, some Maha Nikaya monks have campaigned for the creation of a separate Maha Nikaya patriarch, as recent Thai supreme patriarchs have invariably been drawn from the royalty-supported Dhammayuttika Nikaya.