Which term is used for the condition of unusually high sensitivity to a substance?

The term that refers to the condition of unusually high sensitivity to a substance is "Allergy." An allergy occurs when the immune system mistakenly identifies a harmless substance as a threat and overreacts to it, leading to various symptoms that can range from mild to severe. This heightened response is not just an unpleasant reaction but can also trigger serious health issues such as anaphylaxis in extreme cases.

Allergic reactions typically involve substances known as allergens, which can include food, pollen, pet dander, and certain medications. The distinguishing factor in an allergy is the immune system's involvement, which is absent in conditions like intolerance, where the body's response is not immune-mediated. While intolerance can lead to discomfort and gastrointestinal issues, it does not involve the immune response that characterizes an allergy.