It's το μέρος.

It is not. There are greek neuter nouns with -ος in singular and -η in plural.

Κομμάτι=1) piece, 2) track (song)

Μέρος=1) place, 2) part (also of a book, movie etc)

Τμήμα=1) section, 2)department, 3) a more formal version of "μέρος" (usually when not referring to a book or movie).