Slump
A measure of how fluid fresh concrete is, expressed in inches of subsidence when a standard cone of concrete is unmolded. Higher slump means wetter, more flowable concrete.
What does slump mean?
Slump describes consistency only, not quality or strength. Two loads with identical slump can differ enormously in strength depending on whether the fluidity came from water or from a water-reducing admixture. Contractors want higher slump because it is easier to place and screed; engineers resist it because the cheapest way to get it is adding water.
In practice
Ordering concrete means naming a slump alongside a strength, and a load that arrives stiffer than ordered is the most common source of chute-side arguments.