to be on the point of doing something, or set to happen: to be about to, be going to, be destined to
At its core this word points forward in time. It takes another verb and says that the action has not happened yet but is on its way: someone is about to do something, or something is set to take place. Whether that nearness is a matter of mere minutes (a ship about to sail, a man about to die) or of God's long-settled plan (what is destined to come), the word itself simply marks the action as still ahead and on its way.
Because of this, the same word slides easily between shades that are really one idea. Sometimes it stresses how soon: the thing is right on the edge of happening. Sometimes it stresses purpose or intent: a person means to do this. And sometimes it stresses certainty or destiny: this is fixed to occur because God has determined it. The reader tells which shade is in view from the context, not from the word, which is why translators move freely between "about to," "going to," and "destined to."
When this forward-looking word is used on its own as a name, it gathers all of this into a single phrase for the future itself: what is to come. From there it becomes a fixed expression for the world to come, the age to come, the wrath to come, the glory to come. The same orientation toward what lies ahead is now naming an entire coming reality rather than a single act.