The simplest isomorphism is equality: if two things are equal then they are actually the same thing \(and so not actually two things at all\)\. Anything is obviously indistinguishable from itself under whatever measure you might use \(it has any property in common with itself\) and so regardless of the theory or language, anything is isomorphic to itself\. This is represented by the identity \(iso\)morphism\.
"identity" is probably not a sufficiently specific link; I'd go for math_identity, probably.