Tensile strength
Concrete's resistance to being pulled apart, which is roughly a tenth of its compressive strength. This weakness is the entire reason reinforcing steel exists.
What does tensile strength mean?
Because concrete cracks readily in tension, structural design assumes the concrete carries compression while steel carries tension. Tensile capacity also explains restraint cracking: as concrete shrinks and is restrained by subgrade friction or adjoining construction, it develops tension it cannot resist. Increasing compressive strength raises tensile strength only modestly and can actually make shrinkage cracking worse.
In practice
Every control joint on a slab exists because concrete's tensile strength is not enough to resist its own shrinkage.