To quote from Jordan's Traité des substitutions et des équations algébriques (1870), p. 56 (https://archive.org/stream/traitdessubstit00jordgoog#page/n79/mode/2up):
"§ 67. A group Γ is called "isomorphic" to another group G if one can establish between their substitutions [i.e., elements] a correspondence such that: (1) each substitution of G corresponds to a single substitution of Γ, and each substitution of Γ [corresponds] to one or several substitutions of G ; (2) the product of any two substitutions of G corresponds to the product of their corresponding [substitutions in Γ ], respectively.
The isomorphism will be called "mériédrique" if several substitutions of G correspond to only one substitution of Γ, "holoédrique" in the contrary case."
Thus an "isomorphisme mériédrique" corresponds to a modern "homomorphism", whereas an "isomorphisme holoédrique" corresponds to a modern "isomorphism".
The English equivalent of the French term "holoédrique" would be the crystallographic term "holohedral" (having all of the symmetry planes that are required by the symmetry of a crystal system). The equivalent of "mériédrique" would be be the crystallographic term "merohedral" (having only a fraction of the symmetry planes that are required by the symmetry of a crystal system). Thus, in the case of "holoédrique", the correspondence between the elements of the groups Γ and G is one-to-one and thus isomorphic, whereas in the case of "mériédrique" the correspondence is not one-to-one and therefore homomorphic.