inertial defect

https://doi.org/10.1351/goldbook.I03028
Difference of principal moments of inertia Δ = I C- I A- I B. For planar molecules it is close to zero and for non-planar molecules it is negative.
Source:
Green Book, 2nd ed., p. 23 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)