unerwünscht
unwanted
Not wanted or not welcome; something one would rather not have.
Eine der unerwünschten Nebenwirkungen des Medikaments ist Müdigkeit.
One of the unwanted side effects of the medication is fatigue.

unwanted
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unwanted
Not wanted or not welcome; something one would rather not have.
Eine der unerwünschten Nebenwirkungen des Medikaments ist Müdigkeit.
One of the unwanted side effects of the medication is fatigue.

The word is made up of the prefix 'un-' (not) and 'erwünscht' (desired). 'Erwünscht' is the past participle of the verb 'erwünschen' (to desire/wish for) and is related to 'der Wunsch' (the wish). Something 'Unerwünschtes' is therefore literally something 'not desired'.
Bitte keine unerwünschte Werbung in den Briefkasten werfen.
Please do not put unwanted advertising in the mailbox.
'Unerwünscht' (unwanted) is often used to describe negative consequences or disturbances. Common combinations are 'unerwünschte Nebenwirkungen' (unwanted side effects) in medicine, 'unerwünschte Folgen' (unwanted consequences), or 'unerwünschter Besuch' (unwanted visitors).
Der plötzliche Regen war ein unerwünschter Gast auf unserer Gartenparty.
The sudden rain was an unwanted guest at our garden party.
This adjective is often used in a more formal or factual context, for example in medication package leaflets, technical instructions, or official notices.
Das Update soll unerwünschte Zugriffe auf das System verhindern.
The update is intended to prevent unwanted access to the system.
welcome
'willkommen' and 'unerwünscht' are opposites. 'willkommen' means that a person, thing, or development is wanted and received positively. 'unerwünscht' means the opposite: you do not want something, or it is not allowed or not appropriate. Learners may confuse the words because both often occur in similar situations, for example with guests, behavior, or changes. Pay special attention to the prefix 'un-': it turns the positive 'willkommen' into a negative meaning.

Laute Musik ist im Ruhebereich des Zuges unerwünscht.
Loud music is unwanted in the quiet area of the train.
Für die Planung sind neue Ideen immer willkommen.
New ideas are always welcome for the planning.