What is the legal definition of a religion?

Answer 1

To my knowledge, there is no US or international law directly defining "religion". After some searching, I've found that a legal definition for the US can be paraphrased from Kitzmiller v. Dover Area School District.

A religion is a set of beliefs that "violates the centuries-old ground rules of science by invoking and permitting supernatural causation" and/or is not "limited to the search for natural causes to explain natural phenomena."

To define the context, the first quote was saying that the concept of intelligent design is religious because it "violates the centuries-old ground rules of science by invoking and permitting supernatural causation". The second quote was saying that science, in contrast to intelligent design, is "limited to the search for natural causes to explain natural phenomena."