I think it is one of those terms that means different thinngs to different folks. As noted, every transformer with an independant secondary is an "isolation" transformer. When I see the term used most often it is for a same voltage primary-secondary where they wanted the secondary isolated oftrn for some electronic equipmemt to prevent ground loops, etc..
There are transformers required for pool lighting that have "isolated" secondaries. They have more insulation/spacing/breakdown voltage between the primary and secondary than is normal.
So to answer the question you have to tell us WHAT the application is so we can tell if the specifier is just talking about a regular landscape transformer and not an auto transformer or there is water involved like a pool or fountain that requires a special transformer with an isolated secondary.
If the spec just calls for a UL 1838 transformer, I would say it's nothing special, just a regular landscape lighting transformer that inherently is isolated.