What is the exact meaning of the term 'isostructural' molecules? Wikipedia:

Isostructural


  Isostructural chemical compounds have similar chemical structures.


Chemical Structure


  A chemical structure determination includes a chemist's specifying the molecular geometry and, when feasible and necessary, the electronic structure of the target molecule or other solid. Molecular geometry refers to the spatial arrangement of atoms in a molecule and the chemical bonds that hold the atoms together, and can be represented using structural formulae and by molecular models;[citation needed] complete electronic structure descriptions include specifying the occupation of a molecule's molecular orbitals.


My Doubt

Given that, while practicing, I encountered a question asking whether $\ce{NO3-}$ and $\ce{SO3}$ are isostructural or not. If we go as per the geometry, they are isostructural ($\ce{sp^2}$ hybridization). But if we consider the electronic structure, the former has bond order 4/3, the latter has 2 (assuming $\ce{S=O}$), and so may be, they are not isostructural. (In addition, they also have different total number of electrons.) This (that they are not isostructural) is the answer.

So, this question. Is the term defined or is it used without any strict definition?
Two molecules are isostructural if they have a similar number of atoms arranged in a similar structure (spatial coordinates). In your example, both $\ce{NO3^-}$ and $\ce{SO3}$ have same number of atoms, and they have similar spatial arrangement as well. So they are isostructural. Sometimes using hybridization of the central atom works in finding out, but not always. For example $\ce{SCl3}$ and $\ce{BF3}$ are isostructural, even though the former is $\text{sp}^3\text{d}$, while the latter is $\text{sp}^2$.