“Server” is a colloquialism. As used in casual speech, it’s a system that serves something. If you can access anything the system offers remotely, it’s serving to you and therefore is a server.

Long before I really got into IT, my mom’s laptop had an internet connection it shared. That was a server.

After that but before I setup my first Linux system, my brother and I were sharing files from our desktops. We were both servers (and clients).

A server is just something that serves something.