The term presumption of constitutionality is a legal term used by courts while they interpret and apply a law passed by the legislature. Legislature while enacting a law makes sure that the law doesn't violate the provisions of the constitution. Thus, there is always a presumption in favour of the constitutionality of legislation unless it ex facie violates the fundamental rights mentioned under Part III of the Constitution of India. If the provisions of the law are construed in such a way that they imbibe the spirit of the constitution, however, a second interpretation of the provision is unconstitutional the court would then lean towards the former interpretation. 

Justice K Ramaswamy, in the case of ML Kamra v. New India Assurance (1992), observed that "the court should not interpret the statutory provisions unless the language of the provision is ambiguous"