Two crystals are said to be isostructural if they have the same structure, but not necessarily the same cell dimensions nor the same chemical composition, and with a 'comparable' variability in the atomic coordinates to that of the cell dimensions and chemical composition. For instance, calcite CaCO3, sodium nitrate NaNO3 and iron borate FeBO3 are isostructural. One also speaks of isostructural series, or of isostructural polymorphs or isostructural phase transitions.
The term isotypic is synonymous with isostructural.
Section 3.3 of International Tables of Crystallography, Volume D