"So what is the decisive moment? To put it simply it is the moment, the perfect moment. A single moment in time that is happening right before your eyes, that may last for only a fraction of a second, in which all the elements of the perfect photograph are combined. A beautiful balance of elements that may include the light, subject matter, colour, tones, contrast, lines, movement, expression, and the proper composition, all in a perfect moment of harmony.
It is usually a fleeting moment that has passed you by, even before had a chance to think about photographing it. In fact it is not something you think about at all, it is something you see and feel within a moment, and are hopefully photographing it as it happens. If you are thinking about it, then you have already missed the decisive moment.
Is the decisive moment only for the unpredictability of real life street photography? Absolutely not, the decisive moment transcends that notion and can be found in many fields of photography. You can have a decisive moment while shooting a portrait, a fashion picture, reportage, or in sports photography, because of the emotion, movement, and the unpredictability of people. I do think that people being in the photograph is a very important element to a photograph having the quality or feeling of a decisive moment, but it is not a absolute requirement."Anticipating The Decisive Moment By PAUL KERINS
Note: wow... this is really the decisive definition about the decisive moment...