"危険" (きけん) means danger or hazard. It refers to the possibility of suffering harm or injury. It's the opposite of "安全" (safety). You should always be aware of "危険" and take precautions to avoid it. For example, "この場所は危険です" (This place is dangerous).

害や怪我をする可能性のある状態です。危ないこと。例：危険な場所。