Leukocytosis is an abnormally high level of the combination of all leukocyte (i.e., white blood cell) types. It may be due to a rise in all subtypes, or to a rise in one type; there are specific terms for a selective rise, such as eosinophilia or lymphocytosis. An abnormally low count is called leukopenia.
The normal count can vary among laboratories, but a typical normal adult range is 4,500-10,000 cells per microliter. 
Elevated counts suggest infection, inflammation, or a disorder of the blood-forming apparatus. In such cases, the white blood cell differential, giving the relative percentage or absolute counts of the subtype, is needed to characterize the situation.
- WBC count, Medline Plus