Maybe you thought of an SDK-like client/wrapper for calling certain APIs, so it sounds natural to call it an API client?
Here you can check a list of currently OSS API clients (competitors to Voiden - the tool I posted about) https://github.com/stepci/awesome-api-clients Will join the list soon after we go OSS too. :)
I guess "API client" is often shorthand for "API client library" but that seems less of a stretch than using it to mean "App for calling, testing and documenting an API". A quick Github search seems to indicate that the former usage is more common in any case.