Compressive strength
How much crushing force concrete can carry before it fails, expressed in pounds per square inch. It is the headline number in almost every concrete specification.
What does compressive strength mean?
Compressive strength is measured by crushing molded cylinders in a lab, and it correlates loosely with other properties like abrasion resistance and permeability without actually guaranteeing them. Strength is governed primarily by the water-cement ratio and secondarily by cement content, aggregate quality, and curing. Concrete continues gaining strength well past the age at which it is tested.
In practice
When someone orders '4,000 pound concrete' for a driveway, this is the property being named.