Peak expiratory flow rate (PEFR) is a measure of how quickly a person can exhale air from their lungs, and is used to monitor the function of the respiratory system. It is typically measured using a handheld device called a peak flow meter, and can be useful in diagnosing and managing respiratory conditions such as asthma, chronic obstructive pulmonary disease (COPD), and other lung diseases. PEFR values can vary depending on age, height, gender, and other factors, and are often used in conjunction with other measures such as spirometry to assess lung function.