Wikipedia
Sectional density is the ratio of an object's mass to its cross-sectional area with respect to a given axis. It conveys how well an object's mass is distributed (by its shape) to overcome resistance along that axis. For illustration, a nail can penetrate a target medium with its pointy end first with less force than a coin of the same mass lying flat on the target medium.
Sectional density is used in gun ballistics. It is defined either as a projectile's mass divided by its cross sectional area, or as a projectile's mass in pounds divided by the square of its diameter in inches.
During World War II bunker-busting Röchling shells were developed by German engineer August Cönders, based on the theory of increasing sectional density to improve penetration. Röchling shells were tested in 1942 and 1943 against the Belgian Fort d'Aubin-Neufchâteau and saw very limited use during World War II.