directional bonding
Ionic bonding has no sense of direction: a charged sphere pulls equally on every side, so ions just pack together as densely as their sizes allow, like oranges in a crate. Covalent bonding is different: a shared electron pair points, sticking out along a definite direction fixed by the atom's orbitals and insisting on definite angles between bonds. That is directional bonding, the covalent bond's stubborn preference for particular angles, which forces atoms into open, specific arrangements rather than dense random packing.
The classic case is carbon in diamond. Each carbon forms four covalent bonds aimed at the corners of a tetrahedron, 109.5 degrees apart, and no amount of squeezing will let it gather more neighbours: the bonds simply refuse to bend to it. The result is a rigid, wide-open framework with a low coordination number of four and a lot of empty space, held ferociously stiff because deforming it means bending strong directional bonds. Silicon, SiC, Si3N4, and boron nitride follow the same logic: strong, angular, low-coordination covalent networks.
Directionality is the fork in the road that ionic-versus-covalent character sends a ceramic down. Non-directional ionic bonding gives dense, high-coordination, close-packed structures (rock salt, fluorite) whose properties the radius-ratio rules predict well. Directional covalent bonding gives open, low-coordination frameworks that the radius-ratio rules get wrong, but that reward you with extreme hardness and stiffness (diamond, SiC) because there is no easy way for atoms to slip past one another. Most real ceramics are a blend, and the degree of directionality is exactly what the electronegativity difference is quietly telling you.
Diamond versus rock salt: diamond's directional carbon-carbon bonds force coordination 4 and a wide-open, ultra-hard lattice, while NaCl's non-directional ionic bonds allow coordination 6 and a dense, easily cleaved crystal, the same idea of bonding chooses structure at opposite ends of the spectrum.
Directional bonds build open, rigid cages; non-directional ionic bonds pack densely.
Directional does not mean strong in one direction only and weak elsewhere; it means the bond insists on specific angles. That angular stiffness is precisely why covalent ceramics are so hard, yet also why they cannot rearrange to relieve stress, leaving them brittle.