
einzeln
separately
Separate from one another; one person or thing after another, not together.
Die Schüler wurden gebeten, einzeln nach vorne zu kommen.
The students were asked to come to the front one at a time.
individual / separate
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separately
Separate from one another; one person or thing after another, not together.
Die Schüler wurden gebeten, einzeln nach vorne zu kommen.
The students were asked to come to the front one at a time.

individual
Refers to a single, specific person or thing from a group.
Jeder einzelne Fehler im Text muss korrigiert werden.
Every individual mistake in the text must be corrected.

isolated / scattered
Only a few, occurring here and there.
Es gab nur einzelne Proteste, aber keine große Demonstration.
There were only isolated protests, but no large demonstration.
The word 'einzeln' (individual / separate) is derived from the number 'eins' (one). It always describes something that relates to a single unit or person, or that is separate from others.
Jeder Gast bekommt ein einzelnes Stück Kuchen.
Each guest gets an individual piece of cake.
'Einzeln' can stand before a noun as an adjective and is then declined. But it can also be used as an adverb and then remains unchanged.
Jeder einzelne Gast hat ein Geschenk bekommen.
Every individual guest received a gift.
To emphasize that truly every single person or thing is meant, 'einzeln' (individual) is very often combined with 'jeder', 'jede', or 'jedes' (each/every). This combination strengthens the statement.
Ich habe jede einzelne Seite des Buches gelesen.
I read every single page of the book.
In the meaning 'wenige, verstreut' (a few, scattered), 'einzeln' is often used with plural nouns that normally occur in larger numbers. Typical examples are 'einzelne Fälle' (isolated cases), 'einzelne Häuser' (scattered houses), or 'einzelne Regentropfen' (individual raindrops).
Es gab nur einzelne Beschwerden, die meisten Kunden waren zufrieden.
There were only isolated complaints; most customers were satisfied.
only / unique
Einzig and einzeln can both refer to a particular thing, but they emphasize different aspects. Einzig means that nothing else exists; einzeln singles out one thing from a group without specifying the group’s total number. If there is only one error, einzig is the right term; when dealing with a particular error, einzeln is the right term.

Im Bericht prüfen wir jeden Fehler einzeln.
In the report, we examine each error individually.
Im Bericht wurde nur ein einziger Fehler gefunden.
Only one error was found in the report.