  Detailed studies of galaxy formation require clear definitions of the
structural components of galaxies. Precisely defined components also enable
better comparisons between observations and simulations. We use a subsample of
eighteen cosmological zoom-in simulations from the NIHAO project to derive a
robust method for defining stellar kinematic discs in galaxies. Our method uses
Gaussian Mixture Models in a 3D space of dynamical variables. The NIHAO
galaxies have the right stellar mass for their halo mass, and their angular
momenta and S\'ersic indices match observations. While the photometric
disc-to-total ratios are close to 1 for all the simulated galaxies, the
kinematic ratios are around ~0.5. Thus, exponential structure does not imply a
cold kinematic disc. Above log(M*)~9.5, the decomposition leads to thin discs
and spheroids that have clearly different properties, in terms of angular
momentum, rotational support, ellipticity, [Fe/H] and [O/Fe]. At log(M*)<9.5,
the decomposition selects discs and spheroids with less distinct properties. At
these low masses, both the discs and spheroids have exponential profiles with
high minor-to-major axes ratios, i.e. thickened discs.
