The vesting of residuary powers in the central government by the Constitution of India indicates that India is a quasi federal nation.

In a federal system, like the United States, the powers are divided between the central government and the regional states. Each level of government has its own spheres of authority and autonomy. However, in a unitary system, all powers are concentrated in the central government, with little or no power given to regional states.

In the case of India, residuary powers refer to those powers that are not specifically assigned to either the central government or the state governments. These powers are given to the central government, which suggests a unitary element in the Indian polity. However, India cannot be considered purely unitary as the states do have some degree of autonomy and separate powers.

Therefore, the term "quasi federal" is used to describe the unique nature of India`s political system. It is a combination of federal and unitary features, where power is shared between the central government and the state governments, but the central government holds the authority over residuary powers.