This is the 183rd most frequent Icelandic word.
eina
The Icelandic word "eina" means "the only" or "sole" (feminine singular form).
In this sentence, 'eina' is the female singular accusative form of the numeral 'one', agreeing with 'vinkonu' (female friend).
Ég á eina vinkonu.
I have one female friend.
Here, 'eina' is used as an adjective meaning 'only', describing 'bíóið' (the cinema).
Þetta er eina bíóið í bænum.
This is the only cinema in town.
In this example, 'eina' is the feminine singular accusative form of the numeral 'one', modifying 'köku' (cake).