Recent experiments show that the relative dielectric constant of water confined to a film of nanometric
thickness reaches a strikingly low value of 2.1, barely above the bulk’s 1.8 value for the purely electronic
response. We argue that is not a well-defined measure for dielectric properties at subnanometer scales
due to the ambiguous definition of confinement width. Instead, we propose the 2D polarizability α⊥ as the
appropriate, well-defined response function whose magnitude can be directly obtained from both measurements and computations. Once the appropriate description is used, understanding the interplay between electronic and ionic contributions becomes critical, contrary to what is widely assumed. This highlights the importance of electronic degrees of freedom in interpreting the dielectric response of polar fluids under nanoconfinement conditions, as revealed by molecular dynamics simulations.