Contraction of herunter.
- (colloquial) down, down here, downstairs (towards the speaker)
- Er kommt jetzt runter. -- He's coming downstairs now.
- (colloquial) down, down there, downstairs (away from the speaker)
- Er geht runter zu den andern. -- He's going downstairs to the other people.
Unlike the standard language, colloquial German does not distinguish the meanings of hinunter, hinab ("down there, away from the speaker") and herunter, herab ("down here, down towards the speaker"). Runter is used for both meanings.
- runter in Duden online