Rashi on Exodus 7:1/רש"י על שמות ז׳:א׳

1נתתיך אלהים לפרעה This signifies I have made thee a judge and castigator — to castigate him with plagues and pains.

2יהיה נביאך [AARON] SHALL BE THY PROPHET — This must be understood as the Targum takes it: thy interpreter. Similarly, wherever this term of נבואה is mentioned it refers to a man who publicly proclaims and utters to the people words of reproof. It is of the same derivation as, (Isaiah 57:19) “utterance (ניב) of the lips”; (Proverbs 10:31) “it utters (ינוב) wisdom”; (I Samuel 10:13) “he made an end of proclaiming (התנבות)”, which is in the book of Samuel. In old French we call him predicar; English preacher.