As others have stated matrix and XMPP are good options. I myself am an admitted matrix fan boy, so i lean heavy that way....but the choice is yours. For matrix,

As others have stated matrix and XMPP are good options. I myself am an admitted matrix fan boy, so i lean heavy that way....but the choice is yours. For matrix, i know that beyond the mobile and web browser clients, there are several regular, (non-browser), good ol' fashion desktop style applications (scroll down on this url: https://matrix.org/clients/ https://matrix.org/clients/) for linux, windows and Mac operating systems. The story is likely similar on XMPP side, but i have not used xmpp in many years.