In English there are a few terms used to refer to a brief and unspecified amount or point in time. Instant, moment, and the figurative senses of second and minute are all used to refer to a single basic concept, and are to some extent interchangeable. All four of the following are common, and any variation in meaning is slight.
While minute in this context has less a sense of urgency or brevity than the others and instant has the greatest sense of urgency, it would be unusual for a person to make a distinction between them, and highly unusual for a person to be confused because one of the terms was used instead of another.
Other, less general or common terms or expressions that can be used include jiffy, two tics, or the blink of an eye.
A phrase expressing a similar idea of time is the open-ended phrase "before you can say ****" where **** indicates any word, name, or phrase.
Instant, moment, and the figurative sense of second all share a sense of atomicity—that is to say, they cannot meaningfully be subdivided, because they represent as small a moment of time as can usefully be thought of in a particular context.
Instant, moment, and the figurative senses of second and minute can, along with most measures of time, refer both to an amount of time and to a fixed point in time. Unlike other measures of time, however, these four terms in such usage almost always refer to a moment of synchronicity or coincidence, as in "the instant I put the phone down, it rang," "the moment I got in the door, I was accosted by three needy children," "the minute he stopped topic, he realized there was no one around him."
Unlike the other of these four terms, moment is commonly used to refer a potentially long period of time (days or years) that is small or not usefully divisible within the scale of time it is being used in. An example might include: "there was a moment in the Sixties when it seemed like the government was listening, but that's all gone now." Moment is also distinct in that it can be modified by the word brief; second or minute rarely are, and it would be very odd to say "brief instant."
Instant, in comparison with the other terms, emphasizes the brevity or atomicity of the amount of time. Moment or second, in contrast, can be used in situations where a clear amount of time is passing, and its shortness is not especially emphasized: "take a moment to fill this survey out," or "think about it for a few seconds before you decide."