a fixed point or stretch of time: an hour, a time of day, or the right moment for something
At its core this word marks time as something with edges. It can name one of the divisions of the daylight, the way we still say "the third hour" or "noon," so it works like our clock reading even though the ancient day was measured from sunrise rather than from midnight.
From that it stretches in two directions. Pointed at length, it shrinks to a brief span, a little while, the short window in which something happens or fails to happen. Pointed at significance, it swells into the appointed time, the moment a thing is set to occur, and in the Gospels and Revelation this becomes the loaded "hour" of a person's destiny or of God's decisive action.
These are not separate words but one idea seen from different angles: a bounded piece of time, whether you are reading it off the sky, feeling how short it is, or recognizing that it has finally come.