Strong's G1558 — ἔκδικος (adjective)
ekdikos (ek'-dik-os) adj.
1. carrying justice out, i.e. a punisher
[from G1537 and G1349]
KJV: a (re-)venger 
Root(s): G1537, G1349 
Hebrew root: δίκη
Occurrences: 2