1) the definite article, "the" in its masculine, feminine or neuter gender
2) the demonstrative pronoun
Part of Speech: definite article or demonstrative pronoun in all their inflections. The specific part of speech is dependent upon the context
The masculine, feminine (second) and neuter (third) forms, in all their inflections; the definite article; the (sometimes to be supplied, at others omitted, in English idiom): - the, this, that, one, he, she, it, etc.
These two sites give similar information, with the definition from several dictionaries and statistics on the use of the word.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root ὁ; click on the second column to search for that grammatical form of the root ὁ; click on the third column to search for that word and grammatical form; click on the total to search for the root ὁ.