Definition of Establishment clause
That provision of the First Amendment to U.S. Constitution which provides that “Congress shall make no law respecting an establishment of religion or prohibiting the free exercise thereof. ”. Such language prohibits a state or the federal government from setting up a church, or passing laws which aid one, or all, religions, or giving preference to one religion, or forcing belief or disbelief in any religion. Everson v. Board of Education, 330 U.S. 1, 67 S.Ct. 504, 91 L.Ed. 711; McCollum v. Brd. of Education, 333 U.S. 203, 68 S.Ct. 461, 92 L.Ed. 649.
That's the definition of Establishment clause in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.