a fiber-reinforced composite
The high-performance workhorse: thin, very strong fibers (glass, carbon, aramid) embedded in a matrix. Because fibers are strong along their length, aligning many of them lets a light part carry big loads in the fiber direction, like a bundle of drinking straws glued together resists bending far better than the glue alone.
Three levers govern it: the fiber material (which sets the ceiling on stiffness and strength), the volume fraction (how much is fiber, more fiber means stiffer, up to about 60 to 70 percent in practice), and the fiber length and orientation. Continuous aligned fibers give the biggest gains along the fiber axis; short or randomly oriented fibers give smaller but more uniform gains. Load shares by the rule of mixtures.
Fiber composites give outstanding specific stiffness and specific strength (per unit weight), which is why aircraft, wind-turbine blades, and race cars use them. The honest cost: they are strongly anisotropic, strong along the fibers but weak across them and in shear, so parts must be designed and laid up with the loads in mind, and delamination between layers is a real failure mode.
A wind-turbine blade tens of metres long is mostly glass or carbon fibers running along its length in a polymer matrix, so it is stiff and strong against bending yet light enough to spin.
Great specific strength and stiffness, but only along the fibers.
A fiber composite is only strong in the fiber direction; loaded across the fibers it can be weaker than the matrix-fiber bond allows. Strong always needs the question, in which direction?