1.NounThe state of being breathless or out of breath, especially merely temporarily from exertion.
"Climbing eight flights of stairs while the elevator is out of service can easily produce some breathlessness in people whose physical fitness is otherwise not noticeably impaired."
2.NounDifficult or labored breathing, as a symptom or sign of a medical condition.
"Since he'd entered the urgent care center, his breathlessness had only gotten worse, even though he'd been sitting quietly."