
ultra
Expresses an extreme intensification; means 'very', 'extremely', or 'excessively'. It is often used colloquially.
Die Party gestern war ultra cool.
The party yesterday was ultra cool.
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Expresses an extreme intensification; means 'very', 'extremely', or 'excessively'. It is often used colloquially.
Die Party gestern war ultra cool.
The party yesterday was ultra cool.

Refers to the Ultra movement, a group of especially fanatical and organized supporters of a sports team, particularly in football/soccer.
Die Choreografie in der Fankurve wurde von der Ultra-Szene organisiert.
The choreography in the fan section was organized by the Ultra scene.
In the meaning of 'sehr' (very) or 'extrem' (extremely), 'ultra' belongs to youth language and colloquial speech. It is usually avoided in formal or written contexts.
Das neue Videospiel ist ultra schwer.
The new video game is ultra hard.
As an intensifier, 'ultra' stands directly before the adjective or adverb that it intensifies. It is not declined, so its form does not change.
Der Film war ultra spannend.
The film was ultra exciting.
If 'ultra' appears before an adjective like 'gut' (good) or 'schnell' (fast), it means 'extremely'. In the context of sports and fans, often as a noun ('die Ultras') or in words like 'Ultra-Szene' (Ultra scene), it refers to fanatical supporters.
Das Konzert war ultra laut.
The concert was ultra loud.
extreme
Both words can express strong intensification. „Extrem“ is a regular adjective and can be grammatically adapted to a noun, whereas „ultra“ is usually colloquial and remains unchanged when used as an intensifier. Use „extrem“ in neutral or more formal statements as well, and „ultra“ in casual language.

Das war eine ultra schwierige Aufgabe.
That was an ultra-difficult task.
Das war eine extrem schwierige Aufgabe.
That was an extremely difficult task.