In English the definite article is "the" and the indefinite article is "a" or "an"
The Greek is a little more complex as the articles change depending on the gender of the word they are associated with. However the indefinite article in Greek uses the same word as the number "one", hence ένα μάτι
(ΕΝΑ ΜΑΤΙ) means both "an eye" and "one eye".
Gender Definite Article Indefinite Article
Male The O, ο One, a or an ΕΝΑΣ, ένας
Female Η, η ΜΙΑ, μία
Neuter ΤΟ, το ΕΝΑ, ένα