1. exact moment, very short time
it happened in an instant/instantly
'Instant' is a word that can be used as a noun or an adjective. As an adjective, it means something immediate, urgent or pressing. "The use of video cameras will result in immediate ejection from the cinema." As a noun, 'an instant' refers to a short or precise period of time. 'I'll be with you in an instant." "For an instant, I thought she was going to fire me."