Hakim al Umma Maulana Ashraf Ali Thanvi (Allah have mercy on him) wrote,

‘Arrogance (kibr) is to deliberately consider one’s self superior to others in religious or worldly achievements in a manner that reflects that others are inferior.

Hence, it has two components, (1) to consider one’s self superior and (2) to look down upon others.

This is its reality. It is impermissible (haram) and sinful.

Then there is form of it where all components are presents except one component. That is it is not deliberate. It is an involuntary thought. Till this point it is not sinful. However, if one deliberately considers this thought to be fine or even without agreeing to it deliberately keeps (it in his mind), it will become the reality of arrogance and sinful (activity).

Tarbiyet us Salik, volume 3, page 108