The working definition
Anemia is a hemoglobin or hematocrit below the reference range for the patient's age, sex, and altitude. The threshold is a population reference, not a fixed universal number, so the same hemoglobin can be normal in one patient and anemic in another.
Why the number is only the starting point
A low hemoglobin is a measurement of the end result of a process, not a description of the process itself. Reduced red cell production, increased destruction, and blood loss all lower the hemoglobin, and they do so through different mechanisms that require different tests to separate. Treating the hemoglobin value as the diagnosis stops the reasoning exactly where it needs to begin.
One number, several causes
Consider a hemoglobin of 9 g/dL in three patients. In the first, the red cells are small and pale and the reticulocyte count is low, pointing toward iron deficiency. In the second, the red cells are small but the reticulocyte count is normal and the patient is of Mediterranean ancestry, raising thalassemia trait. In the third, the reticulocyte count is high and the bilirubin is elevated, suggesting hemolysis. The hemoglobin is identical; the classification is not.
Common confusion
A normal hemoglobin does not exclude significant anemia in a patient who has lost blood acutely, because the hemoglobin falls only after the lost volume is replaced. The threshold defines anemia at a point in time, not the presence or absence of a bleeding source.