ausserordentlich
extraordinarily
To a very high degree; especially, unusually good, strong, or intense.
Sie hat bei der Prüfung eine ausserordentlich gute Leistung gezeigt.
She delivered an extraordinarily good performance on the exam.

extraordinary
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extraordinarily
To a very high degree; especially, unusually good, strong, or intense.
Sie hat bei der Prüfung eine ausserordentlich gute Leistung gezeigt.
She delivered an extraordinarily good performance on the exam.

The word is made up of 'außer' (outside/beyond) and 'ordentlich' (orderly/proper). It literally means 'outside the order' or 'outside the normal,' which explains both meanings well.
Das Museum besitzt eine ausserordentliche Sammlung seltener Kunstwerke.
The museum has an extraordinary collection of rare works of art.
'Ausserordentlich' can stand before a noun as an adjective or be used as an adverb to intensify a verb or another adjective. As an adverb, it means 'very' or 'especially.'
Das war eine ausserordentliche Leistung.
That was an extraordinary achievement.
This word belongs to a formal or elevated style. It is used mainly in written language or in official contexts.
Der Redner bedankte sich für die ausserordentliche Gastfreundschaft.
The speaker thanked them for their extraordinary hospitality.
The spelling 'ausserordentlich' with 'ss' is common in Switzerland. In Germany and Austria, the standard spelling is 'außerordentlich' with 'ß.'
In der Schweiz schreibt man „Strasse“, in Deutschland „Straße“.
In Switzerland, people write 'Strasse'; in Germany, 'Straße.'
In the meaning 'additional' or 'special,' the word is often found in formal contexts before nouns such as 'Sitzung' (meeting), 'Versammlung' (assembly), or 'Kündigung' (termination/dismissal).
Der Betriebsrat hat eine ausserordentliche Versammlung einberufen.
The works council called an extraordinary assembly.
especially / special
'besonders' and 'ausserordentlich' are often close in meaning when you want to strongly emphasize something. 'besonders' is the more general word and fits in many different situations. It can also simply mean 'special' or 'not everyday'. 'ausserordentlich' sounds stronger and often somewhat more formal. It emphasizes that something applies to a very high degree or is not part of the normal course of events. In everyday language, people usually say 'besonders' when the statement should not sound so strong or official.

Du hast die schwierige Aufgabe ausserordentlich gut gelöst.
You solved the difficult task extraordinarily well.
Der Kuchen meiner Freundin hat mir besonders gut geschmeckt.
I especially liked my friend's cake.