norddeutsch
northern German
Relating to the region of northern Germany, its landscape, culture, or inhabitants.
Das norddeutsche Wetter ist oft windig und wechselhaft.
Northern German weather is often windy and changeable.

northern German
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northern German
Relating to the region of northern Germany, its landscape, culture, or inhabitants.
Das norddeutsche Wetter ist oft windig und wechselhaft.
Northern German weather is often windy and changeable.

The word is simply composed of 'nord' (north) and 'deutsch' (German). So it describes anything that relates to the north of Germany.
Die Backsteingotik ist eine typisch norddeutsche Architektur.
Brick Gothic is a typically northern German style of architecture.
As an adjective, 'norddeutsch' (northern German) is written in lowercase. The related noun for the region, 'Norddeutschland' (northern Germany), is capitalized as a noun.
Er mag die norddeutsche Landschaft sehr.
He really likes the northern German landscape.
Common collocations refer to geography, weather, and food. Examples are 'die norddeutsche Küste' (the northern German coast), 'das norddeutsche Tiefland' (the northern German lowlands), or 'die norddeutsche Küche' (northern German cuisine).
Fischbrötchen sind ein klassischer norddeutscher Imbiss.
Fish sandwiches are a classic northern German snack.
When it refers to people, 'norddeutsch' (northern German) often describes a particular mentality. Typical associations are a direct manner, reserve, or dry humor.
Sein norddeutscher Humor ist nicht für jeden sofort verständlich.
His northern German humor is not immediately understandable to everyone.
northern
'nördlich' and 'norddeutsch' both have to do with the north. But 'nördlich' mainly describes a location or direction: one place is north of another place. 'norddeutsch' refers to northern Germany, meaning the region, the people, the culture, or even the language there. Learners may confuse the words because both begin with 'Nord-'. In practical terms: use 'nördlich' for maps and directions. Use 'norddeutsch' for things that are typical of northern Germany.

Frau Wagner mag die norddeutsche Küche mit viel Fisch.
Ms. Wagner likes northern German cuisine with lots of fish.
Unser Hotel liegt nördlich vom Bahnhof.
Our hotel is north of the train station.