Plain English Meaning
How much an aircraft part is built to resist bending or flexing when forces push on it. A T-tail needs extra rigidity in the tall vertical fin because it carries the horizontal tail at the top, where loads create more leverage.
Full Definition
The structural rigidity engineered into an aircraft component so that it resists bending, twisting, or flexing under aerodynamic and operational loads. In the context of a T-tail, design stiffness refers specifically to the additional strength built into the vertical stabilizer to support the horizontal stabilizer and elevator mounted on top of it.
Context Anchor
Seen in discussions of T-tail aircraft structure, especially the loads placed on the vertical tail by the horizontal tail mounted high above the fuselage.