This is the 5299th most frequent German word.
aïn
"Aïn" is not a standard German word. It may be a misspelling or from another language.
Here 'einzige' originates from 'ein', demonstrating its grammatical inflection and use to describe uniqueness.
Der einzige Schlüssel, den wir haben, ist verloren.
The only key we have is lost.
In this sentence, 'ein' is inflected to 'einen' as the direct object in accusative case.
Ich habe einen Freund getroffen.
I met a friend.
Here 'ein' is used in its indefinite article form to introduce an unspecified apple.