en ordbog in English:

It's a dictionary.

When I don't know a word, I check it in a dictionary.

Use a dictionary: our teacher always said

I have a dictionary in my backpack.

dictionary /ˈdɪkʃənri; US -neri / noun [countable] (pl. dictionaries) 1 to look up a word in a dictionary a bilingual/monolingual dictionary ▶ słownik