Facts
Swami Kesavananda Bharati challenged the Kerala Land Reforms Act and, more broadly, the power of Parliament to amend the Constitution without limit.
Held
By a razor-thin 7:6 majority, the Supreme Court held that Parliament's amending power under Article 368 is unlimited in scope but cannot be used to destroy the "basic structure" of the Constitution.
Basic Structure Elements (illustrative, not exhaustive)
Supremacy of the Constitution; republican and democratic form of government; secular character; separation of powers; judicial review; fundamental rights.
Significance
This remains the most important constitutional decision in Indian history, protecting the Constitution from being fundamentally altered by any parliamentary majority.