Literally: meet – opportunity – reply – change
Alternately: Adapting to the current situation. Playing things by ear. “Facing the current situation and responding to changes.” Specifically, responding appropriately to changing situations, with the knowledge that it is impossible to predict and plan ahead with perfect accuracy.
Notes: Some people may replace 機 with homophone 期 or even 気, but both of these replacements are considered errors.
This compound can be traced back to Chinese antiquity, attributed in turn to the History of the Southern Dynasties, the History of Song, and the teachings (as collected by his disciples) of Song dynasty Confucian scholar Zhu Xi.