Final answer:
The adjective that defines something characterized by sharpness or severity of sudden onset is 'acute'. It is used to describe a severe condition or an intense situation that develops rapidly.
Step-by-step explanation:
The word you are looking for to describe something characterized by sharpness or severity of sudden onset is acute. This adjective is often used in various contexts, such as medicine, to describe a sudden and severe condition, or in general language to describe an intense situation or sensation that has a rapid onset and is often of short duration.
In a sentence, you might say, "The patient experienced an acute pain in her side, prompting an immediate visit to the emergency room." This illustrates the suddenness and severity implied by the adjective acute. Another example could be "His acute awareness of the situation allowed him to act quickly and effectively." This demonstrates how the term can also reference a keenly perceptive state. The term you're looking for is sharp. When used as an adjective, it can describe something that is characterized by sharpness or severity of sudden onset. For example, a sharp pain or a sharp turn in the road.