Instant and Zero hour

Instant noun - A very small space of time.

Zero hour and instant are semantically related. In some cases you can use "Zero hour" instead a noun "Instant".

Nearby Words: instance, instantly, instantaneous, instancy

Zero hour noun - A time or state of affairs requiring prompt or decisive action.

Instant and zero hour are semantically related. Sometimes you can use "Instant" instead a noun phrase "Zero hour".