"Nicator" translates literally as "one who is victorious", (it is related to "Nike") but the real Seleucus used it more as "Conqueror", a title he gave to himself after either Ipsus or Curupedium, his two greatest victories. I don't recall which.
I'm not sure what the exact origin of "Seleucus" as a first name is. I suspect it may have something to do with the moon, due to the similarity with "Selene".