
einmalig
one-time
Taking place, occurring, or being valid only once; not recurring.
Das war eine einmalige Gelegenheit, die so schnell nicht wiederkommt.
That was a one-time opportunity that won't come again anytime soon.
one-time / unique
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one-time
Taking place, occurring, or being valid only once; not recurring.
Das war eine einmalige Gelegenheit, die so schnell nicht wiederkommt.
That was a one-time opportunity that won't come again anytime soon.

unique
Exceptional, fantastic, or incomparably good; unique in its quality.
Die Aussicht vom Gipfel des Berges war einfach einmalig.
The view from the mountain summit was simply unique.
The word is a compound of 'ein' (one), 'Mal' (time), and the ending '-ig'. It literally means 'existing only one time', which is the basis for both meanings.
Für die Anmeldung fällt eine einmalige Gebühr an.
There is a one-time fee for registration.
You can recognize the positive, emotional meaning 'großartig' (fantastic) from the context. While an 'einmalige Zahlung' (one-time payment) happens only once, an 'einmaliges Erlebnis' (unique experience) is an unforgettable, wonderful experience.
Das Konzert gestern Abend war wirklich einmalig!
The concert last night was truly unique!
In the meaning 'nur einmal stattfindend' (taking place only once), the word is often combined with financial or administrative terms. Typical phrases are 'einmalige Zahlung' (one-time payment), 'einmalige Gebühr' (one-time fee), or 'einmaliges Angebot' (one-time offer).
Dieses Sonderangebot ist eine einmalige Aktion und gilt nur heute.
This special offer is a one-time promotion and is valid only today.
In the meaning 'großartig' (fantastic), 'einmalig' (unique) can also stand alone as a positive reaction or exclamation to express enthusiasm.
Wie war dein Urlaub? – Einfach einmalig!
How was your vacation? – Simply unique!
once
Einmal is an adverb and indicates how often a fee is charged. Einmalig is an adjective and describes the fee itself as non-recurring; before a noun, it is inflected accordingly. Here, both can express the same situation from different grammatical perspectives.

Für die Anmeldung fällt eine einmalige Gebühr an.
There is a one-time registration fee.
Die Gebühr für die Anmeldung wird einmal erhoben.
The registration fee is charged once.