Adverb
Verb (する)
Onomatopoeic
1. with a start; in surprise
その言葉にはっとした。I was startled by those words.
2. suddenly (realize, remember, etc.); all of a sudden
それから私ははっと思い出した。Then I suddenly remembered.
3. puffing (air); with a puff
4. suddenly (of a movement or change)
5. standing out; being showy; being gaudy