I am building for a client online certification application. Person will complete a short training (few slides) and after that the person will get the certificate proving completion of the "course". How to call that person (preferably one word)?
Technically, this would be dramatically overstating the certified person's achievements, but within the context of your application or website, you could reasonably refer to them as graduates:
1: a holder of an academic degree or diploma
definition from m-w.com
In this case, the "diploma" is merely the certificate that you issued, but that should be clear enough from the context of the application.
Certificated. As obvious as it seems, that is the word used, for example, for teachers in the U.S. who have earned their teacher's credentials.
Certificated: "a document attesting to the fact that a person has completed an educational course, issued either by an institution not authorized to grant diplomas, or to a student not qualifying for a diploma." Dic.com