Permeability
How easily water and dissolved chemicals move through concrete. Low permeability is the foundation of nearly every durability property.
What does permeability mean?
Permeability is governed primarily by water-cement ratio and curing, and secondarily by supplementary cementitious materials that refine the pore structure. Dense concrete resists chloride ingress, sulfate attack, carbonation, and freeze-thaw saturation simultaneously, which is why durability specifications tend to control water content rather than name a strength. Cracks bypass all of it, which is why crack control belongs in the same conversation.
In practice
When an engineer insists on a low water-cement ratio despite an easy strength requirement, permeability is what they are actually buying.