Let $(C,\Delta,\varepsilon)$ and $(D,\Delta',\varepsilon')$ be coalgebras.

Definition. We will say that coalgebra homomorphism $f:C\to D$ is a coalgebra isomorphism, if there exists a coalgebra homomorphism $g:D\to C$ such that $f\circ g=\mathrm{id}_{D}$ and $g\circ f=\mathrm{id}_{C}$

Remark. Of course every coalgebra isomorphism is a linear isomorphism, thus it is ,,one-to-one'' and ,,onto''. One can show that the converse also holds, i.e. if $f:C\to D$ is a coalgebra homomorphism such that $f$ is ,,one-to-one'' and ,,onto'', then $f$ is a coalgebra isomorphism.

Definition. We will say that coalgebras $(C,\Delta,\varepsilon)$ and $(D,\Delta',\varepsilon')$ are isomorphic if there exists coalgebra isomorphism $f:C\to D$ In this case we often write $(C,\Delta,\varepsilon)\simeq (D,\Delta',\varepsilon')$ or simply $C\simeq D$ if structure maps are known from the context.

Remarks. Of course the relation ,,$\simeq$ ' is an equivalence relation. Furthermore, (from the coalgebraic point of view) isomorphic coalgebras are the same, i.e. they share all coalgebraic properties.