Concise Bible Dictionary:
ὁ κατήγορος. A title given to Satan when he is cast down to the earth, because he had accused the brethren before God day and night (Rev. 12:10; compare Zech. 3:1).
Strong’s Dictionary of Greek Words:
Transliteration:
kategoros
Meaning:
from 2596 and 58; against one in the assembly, i.e. a complainant at law; specially, Satan