From
http://www.merriam-webster.com/dictionary/seminal
[2] containing or contributing the seeds of later development : creative, original
More specifically, I would call seminal article an article that 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 criteria, 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 :)