When to refer to a paper as "seminal"

I would understand "seminal" to indicate that (a) the paper was the the first in some sense, and (b) that it led to a lot of subsequent research. For example, a paper proposes and tests a theoretical idea, and then lots of other people come along and test that idea building on the original study.

It doesn't matter that you are not established in a field. Anyone can use the word "seminal". It's just that using it correctly requires a good understanding of your field and the interconnections of research papers.

Having a lot of citations is necessary but not sufficient to show that a paper has led to subsequent research. Some citations don't mean much on its own.

From the Merriam Webster dictionary:

[2] containing or contributing the seeds of later development : creative, original

More specifically, I would call an article seminal if it was the start of a new field/trend/idea, the work that inspired everything that came after, a starting point.

Therefore, if that article fulfills this criterion, you can say that, regardless of how many citations it has, or who are the authors. For instance, recent seminal work will not have a lot of citations :)

The question is: why should you mention seminal paper altogether?

I would refrain from giving an adjective to the paper.

Firstly, because "seminal" is subjective. Secondly, because it does really add anything relevant. Thirdly, because it may piss off other people that don't find the paper seminal.

There are better ways of recognising the paper's "seminality", e.g. publishing in the field and giving the due credit to the paper.

In any case, the only occasion I would write "seminal" is if I am pretty sure the work is seminal, which does not seem the case here.