Macrocytic is from Greek words meaning "large cell". A macrocytic class of anemia is an anemia (defined as blood with an insufficient concentration of hemoglobin) in which the erythrocytes are larger than their normal volume. The normal erythrocyte volume in humans is about 80 to 100 femtoliters (fL= 10−15 L). In slightly less correct metric terminology which does not use standard volume units, the size may be given in equivalent cubic micrometers (1 μm3 = 1 fL). The condition of having erythrocytes which (on average) are too large, is called macrocytosis.
In a macrocytic anemia the larger red cells are always associated with insufficient numbers of cells and often also insufficient hemoglobin content per cell. Both of these factors work to the opposite effect of larger cell size, to finally result in a total blood hemoglobin concentration that is less than normal (i.e., anemia).
Macrocytic anemia is not a disease in the sense of having a single pathology, but is rather a condition. As such, it is the class name for a set of pathologies that all produce somewhat the same red blood cell abnormality. Many specific pathologies are known which result in macrocytic-type anemias. Some of these produce slightly different sets of appearances in blood cells that are detectable from red and white cell morphology, and others are only detectable with chemical testing.