Any application of any kind that serves to access something that is served externally can be considered a client, it does not need to be on another machine.
Outlook is a desktop application, right? He is an email client. The browser is an HTTP client that renders pages and it is desktop.
I think an application that does not communicate with something else should not be called a client. But what to think. An application that accesses a file is being serviced at least by the file system , so it is FS client, right?
An application need not be a customer of only one thing. And she does not have to be a customer of anything, just some part of it.
A computer that runs on a network and picks up information from somewhere is a network client, right? It does not even have to be an app to call a customer.
The concept of client and server is orthogonal to what the application does overall. Something in her is a client of something external that provides what she needs.
If all that matters to the application is external access then we can even say that the application is a client in general.
If you have a web application that you load into your browser and from there you do not access anything external, Is she a client? What?
Terms exist to explain a concept adopted. If it is irrelevant in a certain situation, you do not have to use the term. If you are going to use the term you have to fully understand what it is because wrong communication is worse than not communication, in most cases, and in general the person only knows too late whether it matters or not .
Useful info: Is using validation via client enough? .