-ος

From Proto-Hellenic *-os, from Proto-Indo-European *-os. Cognate with the Latin masculine -us (from Old Latin and Proto-Italic *-os both in names and adjectives).

-ος • (-os) m (genitive -ου); second declension

2. Added to verbal roots to form an o-grade action noun.

The accent is recessive.

From Proto-Indo-European *-os.

-ος • (-os) n (genitive -εος or -ους); third declension

1. Added to verbal roots to form a noun of result or an abstract noun of action.
   1. Added to the zero-grade.

      ‎ἔπᾰθον (épathon, “I experienced”, aorist) + ‎-ος (-os) → ‎πᾰ́θος (páthos, “experience, emotion”)

   2. Added to the e-grade.

      ‎μείρομαι (meíromai, “to receive as one's portion”) + ‎-ος (-os) → ‎μέρος (méros, “part”)

-ος • (-os)

1. Nominative singular of masculine and feminine second-declension nouns

   ἄνθρωπος m or f (ánthrōpos, “person”)

2. Masculine nominative singular of first- and second-declension adjectives

   ἄλλος m (állos, “other”)

3. Masculine and feminine nominative singular of second-declension adjectives

   ἄδικος m or f (ádikos, “unjust”)

4. Genitive singular of consonant-stem or uncontracted third-declension nouns

   ὀνόματος n (onómatos, “of a name”)

5. Genitive singular of consonant-stem or uncontracted third-declension masculine or neuter adjectives

   παντός m or n (pantós, “of the whole of”)

• -η (-ē)
• -ς (-s)

From Ancient Greek -ος (-os).

-ος • (-os)

1. An ending found in nouns
   1. In nominative singular form of many masculine nouns

      eg: δρόμος (drómos), καλόγερος (kalógeros), etc

   2. In nominative singular form of few feminine nouns

      eg: μήφος (mífos), διάμετρος (diámetros), etc

   3. In nominative, accusative and vocative singular forms of few neuter nouns

      eg: δάσος (dásos), έδαφος (édafos), etc

2. An ending found in majority of adjectives