definite article -> weak inflection
There are several forms of masculine nouns. You have, zum beispiel, the most common form for masculine in Deutsch: der Mann, den Mann, dem Mann, des Mann(e)s.
This form is easy to remember and much easier to signify as masculine when you are not sure.
But some nouns have another form, having the genitive, the accusative and the dative cases different from the nominative case, zum beispiel:
der Drache, den Drachen, dem Drachen, des Drachen;
der Prinz, den Prinzen, dem Prinzen, des Prinzen.
These are also called sometimes the "N-nouns" (guess why)
There are masculine nouns that are mixed from the two former forms. To this group you can add "Name" (pay attemtion to the genitive -ns ending):
der Name, den Namen, dem Namen, des Namens. So, the first "regular" form and the second "N-nouns" form are mixed to the so-called "mixed" form. It is mixed form the "n" ending of the second one and the genitive "s" ending of the first one. i was taught to call it the "mixed form".