I've seen this term thrown around a lot in the textbooks I'm using as well as part of the Cisco IOS sometimes, but they don't give an actual definition for it. Like in the context of "line con (console) 0" or in the message "line protocol is up", or when you're asked to place an ACL on the line when configuring the RO community string for a lab.
Without extra context, does it just refer to the normal, main (Ethernet) connection to the device? The console connection? As opposed to the VTY lines when those are specified?
Line protocol in particular is confusing for me and it's not explained well. What exactly is it?