When to refer to a paper as "seminal" I am writing a paper that relies heavily upon some important work done some 10  years ago. The paper was well received gathering well over a 100 citations and is a must mention within my niche field. I would like to reference the paper as seminal as in "so and so, et. al., in a seminal paper..." but am not sure that would be appropriate. I recognize 100 citations isn't a lot in the scheme of things but at the same time this paper was very important within my niche field.

Under what circumstances can one decide a paper is seminal? Must the person citing it be well renowned? Must the author of the considered paper be well renowned? Is there a citation # that should be exceeded (I really don't like that one, it seems too inflexible)?

Neither I nor my professor are especially well established in the field we are publishing in (this will be my first publication and my professor mainly publishes in another field). Are we qualified to refer to a publication in this field 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.