Final answer:
Polycythemia is an abnormal increase in the number of red blood cells (RBCs) in the blood. It is not an increase in white blood cells. There are various types of polycythemia, including transient polycythemia and polycythemia vera.
Step-by-step explanation:
Polycythemia is an abnormal increase in the number of red blood cells (RBCs) in the blood, not white blood cells. In contrast to anemia, which is a decrease in RBCs, polycythemia is characterized by an elevated RBC count and is detected through an elevated hematocrit. There are different types of polycythemia, including transient polycythemia caused by dehydration or high altitude living, and polycythemia vera, a rare bone marrow disease that leads to excessive production of immature erythrocytes.