an uncovering: the moment something hidden is brought out into the open, especially something God had kept concealed
At its root this word means taking a cover off. It is built from a verb meaning to cover or veil, with a prefix that means away, so the whole word is the act of pulling the veil away and letting what was hidden be seen. It was barely used in ordinary Greek; it became a religious word among Greek-speaking Jews, and the New Testament writers inherited it loaded with that history.
What ties every use together is this single picture of a cover coming off. The cover may be over a future day, over a long-secret plan, or over an insight given to one person, but in each case the word names the unveiling itself, not merely the thing seen. That is why one English word rarely fits: translators reach for "revelation," "appearing," "vision," or "coming," depending on what is being uncovered.
The senses below sort the New Testament uses by what gets uncovered. They are shades of one idea, not separate words. The same writer can move between them in a few verses, because to him the unveiling of the gospel, the unveiling of the last day, and the unveiling given in a vision are all the one act of God drawing back a curtain.