For me, transport encrypted (e.g. TLS) is the bare minimum everything going over a wire should be encrypted with, so encryption in the context of messenger is for me always about E2E. As far as I know, Google and Apple don't even allow apps to use unecrypted HTTP if dev's want them to be in their respective app store.

As a term, e2e encryption is quite short, and is also easy to search for, if someone does not know what it is.