An abnormality of erythrocytes (red blood cells) where, on microscopy, they appear to have a slit through their centre rather than the normal biconcave appearance.
Stomatocytes are usually seen in hereditary stomatocytosis, liver disease, and acute alcoholism. A small number of stomatocytes often form as an artifact resulting from slow-drying of smears.
I noded this because today, in the doctor's office in one of the hospital wards, an intern was asking "what are stomatocytes?" after reading a haematology laboratory report. So I went back and found out. And here it is.
picture at http://www.medschl.cam.ac.uk/mirrors/lmp/000p0280.jpg and a description at http://www.courses.ahc.umn.edu/medical-school/LaMP/5104/atlas/glossary.htm#stromato