Quoting from a Dictionary

Now, after quoting the definition of the word "didaktik", you must not forget to include a footnote number: this number must always be included before the period, never after, and must ideally appear at the end of your quote.

Regarding the footnote itself, here's how it should appear:

¹ Dudenredaktion, Duden, das Fremdwörterbuch : auf der Grundlage der neuen amtlichen Rechtschreibregeln, Mannheim: Dudenverlag, 2007, [+ page(s) of the book where the definition was found].

You must indicate where the definition was found in the book.  If the definition was found in « seiten » 54, 55 and 56, the footnote would look like this:

¹ Dudenredaktion, Duden, das Fremdwörterbuch : auf der Grundlage der neuen amtlichen Rechtschreibregeln, Mannheim: Dudenverlag, 2007, s. 54-56.

Now, for the bibliography (which must be added at the end of your paper), your reference should look like this:

Dudenredaktion.  Duden, das Fremdwörterbuch : auf der Grundlage der neuen amtlichen Rechtschreibregeln.  Mannheim: Dudenverlag, 2007. 1104 S.

Since we do not know the name of the author(s), we will use Dudenredaktion as the official "author".  There are some differences between footnotes and bibliography references: in a footnote, we use commas and a single period at the end of the footnote; in a bibliography reference, we use a period after the name of the author and after the title, then we indicate the name of the city where the book was edited, we use a colon and we indicate the editor's name, we use a comma, we indicate the year the book was published and we use another period.

We finally indicate the book's total number of pages ("S." is for seiten in German; "p." for pages in English and in French) and we end it all with a final period.