Breathing laboriously is often referred to as wheezing, which is a six-letter word. Wheezing is a sound that is made when breathing is difficult due to a narrowing of the airways, usually caused by asthma, bronchitis, or other respiratory illnesses. It is characterized by a whistling sound that is made when air is forced through the narrowed airways. Wheezing can be a sign of a serious medical condition and should be evaluated by a doctor.