Instant (noun)

1. A particular point in time.
2. A very short period of time.
3. An occurrence, event or situation that happens suddenly.

Origin:

From middle english instant, from old french instant, from latin instant- ("present, pressing, urgent"), from instans, present participle of instare ("to stand upon, press upo.n")

Examples:

1. He arrived at that instant.
2. In an instant, the opportunity was gone.
3. The instant the teacher left the room, the students became noisy.
4. At that instant, the baby started to cry.
5. The instant noodles are ready in just a few minutes.