Compression - Compression is the act of pressing together or forcing into less space—constricting or squeezing.
Applied Concept: A vise squeezing together two pieces of wood is also acting as a compressive force.

Expansion is the act of increasing in dimensions, volume, scope, or inclusiveness; enlargement, extension, spread.
Applied Concept: Expansion can be the increase in length or volume of a material, or a body, caused by temperature, moisture, or other environmental condition. Blowing up a balloon is a form of expansion.

Elasticity/rigidity - Elasticity the capability of an object or material to recover its size and shape after deformation. The term elasticity refers to the manner in which solid materials respond to stress.
Applied Concept: Something that is elastic-or has high ‘elasticity’-is capable of being easily stretched or expanded and then resuming its former shape. Most materials—other than rubber, for instance—are only elastic under relatively small deformations. Rigidity is that property of an object or material which resists a change in physical shape. Below: The ball on the left would be described as rigid and the ball on the right would be elastic.

Density - Density is a ratio of weight to volume.
Applied Concept: Though 2’ diameter spheres of steel and cotton may occupy the same volume, but since the steel is a denser material its sphere will have more weight.