Greek: καὶ ἤκουσα φωνὴν μεγάλην ἐν τῷ οὐρανῷ λέγουσαν Ἄρτι ἐγένετο ἡ σωτηρία καὶ ἡ δύναμις καὶ ἡ βασιλεία τοῦ Θεοῦ ἡμῶν καὶ ἡ ἐξουσία τοῦ Χριστοῦ αὐτοῦ, ὅτι ἐβλήθη ὁ κατήγωρ τῶν ἀδελφῶν ἡμῶν, ὁ κατηγορῶν αὐτοὺς ἐνώπιον τοῦ Θεοῦ ἡμῶν ἡμέρας καὶ νυκτός.
The word translated "accuser" in Revelation 12:10 is κατήγωρ: the term is actually used of someone who brings a legal charge before a judge in court. Compare Zechariah 3:1-5.
From Vincent's Word Studies: "The correct form of the Greek for accuser is a transcript of the Rabbinical Hebrew, κατήγωρ. The Rabbins had a corresponding term συνήγωρ for Michael, as the advocate of God's people. The phrase is applied to Satan nowhere else in the New Testament."
From Jurgen Roloff's Revelation commentary: It is surprising that Satan is characterized here with a word like "accuser" (Gr. kategor), which appears nowhere else in the New Testament. In terms of the history of the motif this forms an association with the notion of Satan as the accuser of human beings before the divine tribunal (Job 1:9-11, 2:4-5: Zech. 3:1). This does not mean that the dominant view of Satan in this chapter as God's adversary should be minimized, but rather that a particular aspect of his activity is highlighted: his aim is to destroy the relationship of God with human beings.
Jurgen Roloff. Revelation (Continental Commentary Series) (Kindle Locations 2201-2203). Kindle Edition.
Jurgen Roloff. Revelation (Continental Commentary Series) (Kindle Location 2200). Kindle Edition.
William Mounce's definition for κατήγωρ: "accuser, Rev. 12:10, a barbarous form for κατήγορος*"