Tier-1 cities are top 7-8 Indian cities based on population. Tier-2 and Tier-3 are smaller cities

https://en.wikipedia.org/wiki/Classification_of_Indian_citie...

There is no golden rule, but assume tier-2 cities mayn't have airport, no famous schools, less than 2 or 3 million people living in there.

Most of the tier-1 cities has direct international or national airways connectivity.

Tier-3 cities may not even have a single factory, it is hub for 100s villages, has decent bus-connectivity to nearest tier-2 city.

Apparently there is an official definition: https://en.wikipedia.org/wiki/Classification_of_Indian_citie...