Just as the word suggests- adaptation, to adapt or adjust. So cellular adaptation means the ability of the cells to adjust to extreme changes. Basically, cells change in response to adverse environmental changes. It is considered normal if the adaptation is physiological and abnormal if it is pathological.
For instance, it is normal to have uterine enlargement during pregnancy, while a myocardial hypertrophy to prevent injury as a result of systemic hypertension is abnormal. This is an example of pathologic.
Now if the environmental change/ or stress is too extreme or persists, cellular injury may occur.
There are 5 types of cellular adaptation- atrophy, dysplasia, hypertrophy, hyperplasia, metaplasia.
Also, note that changes in cellular adaptation are reversible.