Plain English Meaning

It is how strongly an aircraft naturally tries to roll back to wings-level after a sideways slip, measured as if all that rolling tendency came from the wings being tilted upward at the tips — even when most of it actually comes from other design choices like wing sweep or a high-mounted wing.

Full Definition

The total roll-stability effect produced by a wing's design features acting together, expressed as the equivalent amount of geometric dihedral (wingtips angled upward) that would produce the same rolling tendency back toward level flight after a sideslip. Sweepback, high wing placement, and actual dihedral all contribute to a wing's effective dihedral.