Beta-agonist: A bronchodilator medicine that opens the airways by relaxing the muscles around the airways that may tighten during an asthma attack or in COPD (chronic obstructive pulmonary disease). Beta-agonists can be administered by inhalers or orally. They are called 'agonists' because they activate the beta-2 receptor on the muscles surroundin...Beta agonist: See: Beta-agonist.