Being enthusiastic about or absorbed in something; being engrossed or devoted to an activity.
彼はゲームに熱中して、宿題を忘れていた。
He was so absorbed in a game that he forgot his homework.
夏は暑さで熱中症に気をつける必要がある。
In summer you need to be careful about heatstroke.
彼女は研究に熱中して時間を忘れることが多い。
She often loses track of time because she's so absorbed in her research.