How about the Constitutional definition? You've got the Establishment clause and the Free Exercise clause.
Establishment clause: government cannot promote one religion. It cannot define a state religion or financially support any single religion.
Free Exercise clause: government cannot impinge on someone's right to practice his or her religion. This is the more controversial of the clauses. What if someone's religion involves ritual sacrifice, multiple spouses, or use of hallucinogenic substances? The courts of the United States are constantly conflicted as to the definition of this clause.
Anyways, that's my interpretation and the legal interpretation of Americans' constitutional freedom of religion.