a moment is a unit of time, usually used as a way to measure a short interval. A moment is equal to a single moment in time; one moment in time can last from an instant to several minutes. It is also typically used when referring to long periods of time, such as 'a moment in history'. Moments can be considered to be independent temporal segments, though they can be linked to each other in some sense.
1. The moment he saw her, he knew she was the one.
2. Everyone waited with baited breath for the moment the movie would begin.
3. I savoured the moment as I took a sip of my coffee.
moment should not be confused with minutes or hours, which are units of time that are much longer than a moment. Also, remember that a moment can refer to an indeterminable period of time, rather than a very specific point in time. So when saying 'a moment of joy' it implies that there is a period of joy, not just a single instance of joy.