Peak expiratory flow (PEF).

Forced expiratory flow at 25 and 75% of the pulmonary volume (FEF25-75%) is defined as the mean forced expiratory flow during the middle half of the FVC and measures average flow rates on an FVC segment that includes flow from medium-to-small airways 

https://www.atsjournals.org/doi/10.1164/rccm.200708-1248OCPdf:https://www.atsjournals.org/doi/pdf/10.1164/rccm.200708-1248OC