@shanie it’s not "my" definition. Client is well-defined, in this context it’s client for mstdn-compatible c2s API.
Which, in the context of a webbrowser (which is merely an HTTP client), would be a client-side application running in the webbrowser. There’s more than one, you know.
@mirabilos So, like, are you asking what browsers this supports? And does the OP not answer what browsers this is an extension for or something? You’re wording is in a way which is making this way too difficult to answer.