What does abnormal red blood cell mean?

Depends: This depends on what is being called abnormal. There are many possibilities, ranging from red cell count, red cell size, or red cell morphology. And on occasion, some may use this terminology to refer to an abnormal hemoglobin.

Many things: A not so specific term that could mean low count, pale in color or lacking iron or hemoglobin, irregular instead of round shape, etc. Often in CBC, we see red cell count and indices: RBC, Hemoglobin, mean cell volume, mean cell hemoglobin with a normal range compared to a patient. Abnormal red blood cell may mean low or high in any of indices. In blood smear, these abnormalities are seen too.