Split-second (noun)

1. A very short period of time, typically a fraction of a second.
2. A moment of great immediacy or urgency.

Origin:

Early 20th century: from split + second.

Examples:

1. He made the decision in a split-second.
2. The accident happened in a split-second.
3. He had to react in a split-second to avoid the car.
4. The runner had to make a split-second decision to change direction.
5. In a split-second, the situation had changed and he had to act fast.