n. A similarity of crystalline form: as, between substances of analogous composition or atomic proportions, as the members of a group of compounds like the sulphates of barium, strontium, and leadn. between compounds of unlike composition or atomic proportions.n. In biology, the state or condition of being different in ancestry, but alike in appearance. See isomorphic, 2 .n. In group-theory, the state or character of being isomorphic.