As a noun zen
is (religion) a denomination of buddhism.
As an adjective zen
is (religion) pertaining to this denomination of buddhism.
As a preposition nen is
Other Comparisons: What's the difference?
(religion) A denomination of Buddhism elaborated in Japan.
(informal) A philosophy of calm, reminiscent of that of the Buddhist denomination.
Usually capitalized in all senses, almost always when talking about the denomination proper (compare (Catholicism)), usually but less often when referring to a looser sense of the philosophy or calm: “That’s very zen.” versus “She studies Zen Buddhism.”
(colloquial) Extremely relaxed and collected.