A material for which a given material property is the same in every direction - i.e., the material properties do not depend on the direction in which the material is tested. Only two elastic constants (i.e., any combination of modulus of elasticity, shear modulus, or Poisson's ratio) are needed to fully characterize an isotropic material. The practical consequence is that, for an isotropic material, the wave speed is the same in every direction. Thus, for a given horn design and specified frequency, the horn's tuned length will not depend on which direction the horn was machined from the original material.
Aluminum is nearly isotropic.