Orthopnea is breathlessness on lying down.
When taking a medical history, you gain a rough idea of the degree of orthopnea the patient has by asking them how many pillows they sleep on at night.
Orthopnea can be caused by various conditions, but a common cause of orthopnea is pulmonary oedema. Lying down exacerbates breathlessness partly because the weight of the abdominal contents pushes against the diaphragm when the patient is supine, increasing resistance to breathing.
Note: This is spelt as orthopnoea in areas using British English (as opposed to American English).