What is an overt act?

The term overt act makes 126 appearances in the Georgia indictment. E.g., “The speech was an overt act in furtherance of the conspiracy.”

Black’s Law Dictionary (9th ed.), ed. Bryan Garner, gives two meanings from criminal law: “1. An act that indicates an intent to kill or seriously harm another person and thus gives that person a justification to use self-defense,” and “2. An outward act, however innocent in itself, done in furtherance of a conspiracy, treason, or criminal attempt.” And N.B: “An overt act is usu. a required element of these crimes.”

[Black’s is now in its eleventh edition. But the ninth is what I could get my hands on.]