to cry out: to raise a loud, urgent voice that forces itself on those who hear it.
At its root this is the loudest of the New Testament's words for crying out. It started as an imitation of harsh, raw sound (the croak of a raven), and that edge never quite leaves it: when someone does this, the volume itself is part of the point. The cry may be a wordless scream of terror or pain, a chant roared by a crowd, or a solemn announcement made at the top of the voice, but in every case it is a sound that demands to be heard and cannot be ignored.
These are not separate words pretending to be one. They are the same act aimed in different directions. A demon shrieking, a mob chanting, and a prophet declaring all share the same core: a voice pushed to full force because the moment calls for it. The differences below are differences of who is crying and why, not differences in what the word means.
Because the volume carries so much weight, the word often marks the high emotional or spiritual pitch of a scene. The Gospels use it for desperate people begging Jesus for help, for the shouting that drives his trial, and for his own great cry from the cross. Paul uses it for the deepest prayer of the believer, the Spirit crying out to God as Father. Revelation fills the sky with it, angels and voices announcing what God is about to do.