Final answer:
Orthopnea is the term that describes difficulty breathing when a client is lying flat. It is commonly seen in individuals with heart failure or pulmonary edema.
Step-by-step explanation:
The term that describes difficulty breathing when a client is lying flat is Orthopnea. Orthopnea is a condition where individuals have trouble breathing when they are in a supine position, and they usually have to prop themselves up with pillows or sit upright to breathe comfortably. This is often seen in individuals with heart failure or pulmonary edema.