A biconcave disc is a geometric shape resembling an oblate spheroid with two concavities on the top and on the bottom.
Biology
Erythrocytes are in the shape of a biconcave disc. It has been suggested that this shape is optimal for surface area, but this hypothesis has not been widely accepted. In addition, it has been suggested that the biconcave shape is needed to optimise the blood's flow properties.
References
Biconcave disc Wikipedia(Text) CC BY-SA