A 'mesh' is just a variety of describing a network, one which is generally self-organizing. As you can guess, the definition is extremely flexible.
#Tox is an example of a #p2p #mesh used to create a message exchange network focused on #privacy. It uses e2ee, and allows exchange of instant messages, as well as e2ee video calls. There are multiple implementations, rather like the Mastodon protocol has multiple platforms.
https://tox.chat/
https://en.wikipedia.org/wiki/Tox_(protocol)