Originally a branch of Mahayana Buddhism, dating back to Old Earth, that emphasizes meditation, self-contemplation, and intuition over traditional forms of worship. It was practiced originally in Asian countries like China, Japan, and Vietnam, but became very popular among the CisLunar States and some Oortists and outsystem explorers. A successful memeticity during the First Federation period, it's influence was to decline in some of the Inner Sphere polities with the rise of the Archailectic noetopics. But in the Negentropy Alliance, Sophic League, Terran Federation, and FAS it remains quaintly popular, fashionable, and widespread to this day. Also a popular religion in some Outer Volume polities. In each noetic and region of space it has developed along its own lines. See also Pozen. The following are only a few examples, but it should be pointed out that in each empire many forms of Zen can co-exist:
Original Zen - (Old Core Inner Worlds, Terran Federation, FAS, Sophic League, Outer Volumes) Enlightenment is defined as a "gateless gate." Rather than subscribing to the notion of achieving a reward at the end of a long struggle, an enlightened Zen practitioner simply sees everything as it is, as it's always been. Depending on the school and the orientation of the teacher, either meditation, koans, or both may be emphasized.
Orthodox Zen - (Terran Federation, some Old Core Inner Worlds). Seeks to stick as close to original Zen as possible, avoiding PoSenist influence.
Ultra-orthodox Zen - (Terran Federation only). A popular memeticity among some Terran Federation bureaucrats. Even more fundamentalist than Orthodox Zen, and a running joke among all other Zen schools.
Negentropist Zen - Also simply "Zennism". Emphasizing a minimalist and civic aesthetic through quiet meditation and carefully cultivated Zen gardens.
Satori School - popular among some Sophic Zen sects, and deriving all the way back to the Rinzai tradition of Old Earth Japanese Zen, the accent is less on meditation and more on questioning perceived assumptions of logic in words, sentences, ideas, and memes in general, in order to break through to a purely trans-conceptual state of intuitive knowledge (satori).
Meditation School - also popular in the Sophic League, emphasizes quieting the mind, and long periods of monastic or contemplative retreat. The Meditation School is popular also in the Negentropy Alliance.
Trader Zen - developed among some relativist traders of the FAS, focuses on both unity and self-denial, and on a more creative approach to Zen Gardening than that taken by the Negentropists.