What is the medical term for a feeling of being unusually tired or weak?

The term that best describes a feeling of being unusually tired or weak is "fatigue." This term encompasses the overall sense of exhaustion and decreased energy that can result from various factors, including physical exertion, mental strain, or medical conditions. Fatigue is often referred to in both clinical and everyday contexts to indicate a lack of vitality or a heightened sense of weariness.

While "asthenia" refers to a specific form of weakness or lack of energy often seen in clinical settings, it does not capture the broader sense of tiredness that "fatigue" does. "Tachypnea" is a term used to describe rapid breathing and is unrelated to tiredness or weakness. "Lethargy" can imply a state of sluggishness or lack of energy, but it may also suggest impaired alertness or responsiveness, which is not exactly the same as the general feeling of tiredness that "fatigue" conveys.