Inertia tensor
The 3×3 matrix describing how a rigid body's mass is distributed around its centre of mass — the rotational counterpart to mass.
Where mass says how hard it is to change something's speed in a straight line, the inertia tensor says how hard it is to change its rotation. It ties angular velocity to angular momentum (L = Iω) and torque to angular acceleration. In the body's own principal axes the matrix is diagonal, with three principal moments of inertia; a long rod is easy to spin about its length but tiring to tumble end over end.
A physics engine stores the inverse tensor in the body's own frame — where it never changes — and rotates it into world space each step with I⁻¹ = R·I⁻¹₀·Rᵀ. Without a correct tensor, ragdolls and flying debris spin wrong, and vehicles feel weightless through corners.