Complex sentence: Forced Expiratory Flow (FEF) is the flow (or speed) of air coming out of the lung during the middle portion of a forced expiration, given at specific specific intervals, generally defined by what fraction remains of the functional vital capacity (FVC).
Split into simpler sentences: Forced Expiratory Flow (FEF) is the flow (or speed) of air coming out of the lung during the middle portion of a forced expiration. It can be given at discrete times, generally defined by what fraction remains of the functional vital capacity (FVC).