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.

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' and 'einmalig' are both connected to the idea that something happens only one single time. 'einmal' is an adverb and says how often something happens: once. 'einmalig' is an adjective and describes a thing, appointment, or offer as non-recurring. Learners may confuse them because they look very similar. A practical tip: for the question 'How often?', 'einmal' fits. When describing a noun, 'einmalig' often fits.

Für den Kurs gibt es eine einmalige Gebühr.
There is a one-time fee for the course.
Laura hat den Arzt im Sommer nur einmal besucht.
Laura visited the doctor only once in the summer.