The Doctrine of Basic Structure, propounded by the Supreme Court in the landmark Kesavananda Bharati vs. State of Kerala (1973) case, is a cornerstone of Indian constitutionalism. It posits that while Parliament has the power to amend any part of the Constitution under Article 368, this power is not unlimited and does not extend to altering or destroying the "basic structure" or "fundamental features" of the Constitution.
The Supreme Court has, through various subsequent judgments, identified several elements that constitute this basic structure. This list is illustrative and not exhaustive, meaning the Court can identify new basic features as and when cases arise. These identified elements represent the core values, principles, and institutional frameworks that form the very identity and foundation of the Indian Constitution, and are thus considered inviolable by parliamentary amendment.
(Source: Broad understanding synthesized from Laxmikanth, 'Indian Polity'; D.D. Basu, 'Introduction to the Constitution of India'; M.P. Jain, 'Indian Constitutional Law'; Landmark Supreme Court Judgments)