fwiw, the usual usage of the term (as in the above comment) is not the same as the highly specific government usage that the wiki page described.
The usual usage is just:
Tier 1: Metro cities, the ones you've likely heard of. Mumbai, Bangalore, Delhi etc.
Tier 2: Cities, but much smaller population-wise. These are the cities that are getting newer airports for eg. Most state-capitals would fall here. These count up to around 50 to 100 depending on who you ask. Linked to national highways usually.
Tier 3: Small urban towns. Population is usually a few lakh (1 lakh=100_000) at most. (<300k or so).
It more-or-less corresponds to the X/Y/Z usage on the wikipedia article.