The Lemon test (US law)

Overview
The Lemon test is a legal precedent set by the 1971 Supreme Court case Lemon v. Kurtzman. The test "details the requirements for legislation concerning religion. It consists of three prongs":
 * 1) The government's action must have a legitimate secular purpose;
 * 2) The government's action must not have the primary effect of either advancing or inhibiting religion;
 * 3) The government's action must not result in an "excessive government entanglement" with religion.

If any of these 3 prongs is violated, the government's action is deemed unconstitutional under the Establishment Clause of the First Amendment to the United States Constitution, violating the constitutional separation of church and state.

Reference

 * (Lemon v. Kurtzman)