a herald's public announcement: both the act of proclaiming a message aloud on someone else's authority and the message that gets proclaimed
The word belongs to the world of the public herald (κῆρυξ), the officer who stood where a crowd could hear him and read out a ruler's decree. What he delivered was not his own opinion. It was handed to him, and his one job was to make it heard. That background shapes every use in the New Testament: an announcement of this kind carries weight because of who stands behind it, not because the speaker is impressive.
Greek uses this noun for two tightly linked things, and often for both at once: the act of announcing, and what gets announced. Paul can set his announcing beside his manner of speech, so that the activity is in view, and he can also say the announcement would be empty if Christ was not raised, where the truth of the content is what is at stake. English does much the same with the word "proclamation", which is why most translations do not try to mark the difference.
The verb of the same family (κηρύσσω) names the activity, and the good news (εὐαγγέλιον) names the content by its glad character. This noun sits between them: the announcement considered simply as an announcement. It is also not teaching (διδαχή), which unfolds and explains at length. Announcing declares something and asks for a response.
The one example from before the Christian message shows the pattern plainly. Jesus points to what Jonah announced at Nineveh, a blunt warning that turned the city around, and the Greek Old Testament had used this very noun for the cry Jonah was told to make (קְרִיאָה).