Fascism is an ideology. While the BJP has strong ethnonationalist tendencies, it is no way facist. Statist authoritarianism and larger than life political personas have been very much the default in India since independence.

Fascism is not an ideology, strictly speaking it was just Mussolini's political movement.

Other authoritarian movements or regimes that came after it either called themselves or were called fascist.

Largely speaking it's often used to refer to any authoritarian/totalitarian regime or political movement, especially right-wing ones.