From Online Dictionary of Crystallography
Cristaux isomorphes (Fr). Isomorphe Kristalle (Ge). Cristales isomorfos (Sp). Cristalli isomorfi(It). 同型の結晶 (Ja)
Two crystals are said to be isomorphous if (a) both have the same space group and unit-cell dimensions and (b) the types and the positions of atoms in both are the same except for a replacement of one or more atoms in one structure with different types of atoms in the other (diadochy), such as heavy atoms, or the presence of one or more additional atoms in one of them (isomorphous addition). Isomorphous crystals can form solid solutions.
The notion of isomorphism was discovered by Eilhard Mitscherlich (1794–1863) who found that the crystal forms of salts such as the hydrated potassium phosphates and arsenates or the hydrated potassium copper and iron sulfates were identical (1819, 1820).