Acid reflux means that some acid leaks up (refluxes) into the oesophagus.
Oesophagitis means inflammation of the lining of the oesophagus. Most cases of oesophagitis are due to reflux of stomach acid which irritates the inside lining of the oesophagus.
This is a companion discussion topic for the original article at https://patient.info/digestive-health/acid-reflux-and-oesophagitis