Rouleaux refers to the characteristic appearance of red blood cells forming "a stack of coins." It is usually associated with a high concentration of proteins, which changes the interaction between the erythrocytes.
Rouleaux is French for "roll" as in roll of coins.
The stacking of the red cells can be seen in a wet mount of whole blood. Dilution of the blood with normal saline reduces the protein concentration, and the rouleaux disappears.
