<unk>  is perceived to be difficulty of breathing or painful breathing.  It is a common symptom of numerous medical disorders.<unk>  on exertion (DOE or exertional<unk>  indicates<unk>  that occurs (or worsens) during physical activity.<unk>  is subjective, few people even if they do not have any health problem, get short of breath even with mild activity. Other people may have advanced lung disease but they will not have any symptoms.
mMRC breathlessness scale
Grade 0-no<unk>  except with strenuous exercise, 1-dyspnea when walking on a inclined land or hurrying on level, 2- walks slower than most on level land, stop after 15 minutes of walk on level land, 3-stop after few minutes of walking, 4-dyspnea with minimum activity like getting dressed, too<unk> c to leave the house.
Dyspnea
Hypernea
Apnea
Tachypnea
Orthopnea
Labored breathing
Fatigue  
Fatigue