An onomatopoeia for flaring up or suddenly becoming intensely bright (e.g., a fire flaring up or a sudden bright light).
ストーブの炎がカッと燃え上がった。
The stove's flame flared up suddenly.
朝日がカッと差し込み、部屋が明るくなった。
The morning sun shot in suddenly and the room brightened.
To suddenly fly into a rage or lose one's temper (often used as かっとする).
彼は冗談を真に受けてかっとして部屋を出て行った。
He took the joke seriously, flew into a rage, and left the room.
かっとしてからでは遅いので、一度深呼吸した方がいい。
Once you lose your temper it's too late, so it's better to take a deep breath first.
Used to describe something like eyes or mouth opening suddenly and widely in surprise or shock.
そのニュースを聞いて、彼女はカッと目を見開いた。
On hearing the news, she opened her eyes wide in shock.
驚いて口がカッと開いたままになった。
She was so surprised that her mouth stayed wide open.
An older usage meaning to act resolutely or decisively.
彼はためらわずに決断し、カッと前へ進んだ（やや古い表現）。
He made a decision without hesitation and strode forward decisively (somewhat archaic expression).