Asthma refers to a very common condition causing shortness of breath, cough and wheeziness.
Can be started by exercise, contact with animals, stress, and various other allergies. Not a sign of being unfit. Usually requires some form of inhaler regularly.
Other /More definition:
Asthma refers to a chronic disease that causes constriction of the bronchial tubes, preventing air from passing freely and causing wheezing and difficulty breathing during attacks. A complex condition that affects the small tubes that carry air in and out of the lungs. An asthma trigger is anything that irritates the airways and causes the symptoms of asthma to appear. The usual symptoms are wheezing, a whistling sort of breathing, coughing, breathlessness or a tight feeling in the chest. The lining of the airway swells and produces a sticky mucus. The muscle around the walls of the airways tightens so that the airway becomes narrower. As the airways narrow, it becomes difficult for the air to move in and out - this results in breathing difficulties with a wheezing or whistling noise. A variety of triggers can make symptoms worse, such as exercise, viral infections, tobacco smoke and cold weather. Allergens, such as pollen, the house dust mite, carpets and soft furnishings and animal fur can also trigger asthma.